Merge pull request #24258 from ClickHouse/mvcc_prototype

MVCC prototype for MergeTree

base/daemon/BaseDaemon.cpp

@@ -828,7 +828,6 @@ void BaseDaemon::initializeTerminationAndSignalProcessing()
 
     /// Setup signal handlers.
     /// SIGTSTP is added for debugging purposes. To output a stack trace of any running thread at anytime.
-
     addSignalHandler({SIGABRT, SIGSEGV, SIGILL, SIGBUS, SIGSYS, SIGFPE, SIGPIPE, SIGTSTP, SIGTRAP}, signalHandler, &handled_signals);
     addSignalHandler({SIGHUP}, closeLogsSignalHandler, &handled_signals);
     addSignalHandler({SIGINT, SIGQUIT, SIGTERM}, terminateRequestedSignalHandler, &handled_signals);

docker/test/stateless/run.sh

@@ -139,7 +139,7 @@ pigz < /var/log/clickhouse-server/clickhouse-server.log > /test_output/clickhous
 #   directly
 # - even though ci auto-compress some files (but not *.tsv) it does this only
 #   for files >64MB, we want this files to be compressed explicitly
-for table in query_log zookeeper_log trace_log
+for table in query_log zookeeper_log trace_log transactions_info_log
 do
     clickhouse-client -q "select * from system.$table format TSVWithNamesAndTypes" | pigz > /test_output/$table.tsv.gz &
     if [[ -n "$USE_DATABASE_REPLICATED" ]] && [[ "$USE_DATABASE_REPLICATED" -eq 1 ]]; then

programs/server/Server.cpp

@@ -1639,6 +1639,8 @@ int Server::main(const std::vector<std::string> & /*args*/)
                 server.start();
                 LOG_INFO(log, "Listening for {}", server.getDescription());
             }
+
+            global_context->setServerCompletelyStarted();
             LOG_INFO(log, "Ready for connections.");
         }
 

src/Access/Common/AccessType.h

@@ -102,6 +102,7 @@ enum class AccessType
     \
     M(KILL_QUERY, "", GLOBAL, ALL) /* allows to kill a query started by another user
                                       (anyone can kill his own queries) */\
+    M(KILL_TRANSACTION, "", GLOBAL, ALL) \
     \
     M(MOVE_PARTITION_BETWEEN_SHARDS, "", GLOBAL, ALL) /* required to be able to move a part/partition to a table
                                                          identified by its ZooKeeper path */\

src/Client/ClientBase.cpp

@@ -1129,7 +1129,7 @@ void ClientBase::sendData(Block & sample, const ColumnsDescription & columns_des
             sendDataFromPipe(
                 storage->read(
                         sample.getNames(),
-                        storage->getStorageSnapshot(metadata),
+                        storage->getStorageSnapshot(metadata, global_context),
                         query_info,
                         global_context,
                         {},

src/Common/ErrorCodes.cpp

@@ -617,6 +617,8 @@
     M(646, CANNOT_BACKUP_DATABASE) \
     M(647, CANNOT_BACKUP_TABLE) \
     M(648, WRONG_DDL_RENAMING_SETTINGS) \
+    M(649, INVALID_TRANSACTION) \
+    M(650, SERIALIZATION_ERROR) \
     \
     M(999, KEEPER_EXCEPTION) \
     M(1000, POCO_EXCEPTION) \

src/Common/SystemLogBase.cpp

@@ -11,6 +11,7 @@
 #include <Interpreters/TraceLog.h>
 #include <Interpreters/ProcessorsProfileLog.h>
 #include <Interpreters/ZooKeeperLog.h>
+#include <Interpreters/TransactionsInfoLog.h>
 
 #include <Common/MemoryTrackerBlockerInThread.h>
 #include <Common/SystemLogBase.h>

src/Common/SystemLogBase.h

@@ -23,6 +23,7 @@
     M(QueryViewsLogElement) \
     M(SessionLogElement) \
     M(TraceLogElement) \
+    M(TransactionsInfoLogElement) \
     M(ZooKeeperLogElement) \
     M(ProcessorProfileLogElement) \
     M(TextLogElement)

src/Common/TransactionID.cpp

@@ -0,0 +1,43 @@
+#include <Common/TransactionID.h>
+#include <Common/SipHash.h>
+#include <IO/ReadHelpers.h>
+#include <IO/WriteHelpers.h>
+
+namespace DB
+{
+
+TIDHash TransactionID::getHash() const
+{
+    SipHash hash;
+    hash.update(start_csn);
+    hash.update(local_tid);
+    hash.update(host_id);
+    return hash.get64();
+}
+
+
+void TransactionID::write(const TransactionID & tid, WriteBuffer & buf)
+{
+    writeChar('(', buf);
+    writeText(tid.start_csn, buf);
+    writeCString(", ", buf);
+    writeText(tid.local_tid, buf);
+    writeCString(", ", buf);
+    writeText(tid.host_id, buf);
+    writeChar(')', buf);
+}
+
+TransactionID TransactionID::read(ReadBuffer & buf)
+{
+    TransactionID tid = Tx::EmptyTID;
+    assertChar('(', buf);
+    readText(tid.start_csn, buf);
+    assertString(", ", buf);
+    readText(tid.local_tid, buf);
+    assertString(", ", buf);
+    readText(tid.host_id, buf);
+    assertChar(')', buf);
+    return tid;
+}
+
+}

src/Common/TransactionID.h

@@ -0,0 +1,115 @@
+#pragma once
+#include <Core/Types.h>
+#include <Core/UUID.h>
+#include <fmt/format.h>
+#include <IO/WriteHelpers.h>
+
+namespace DB
+{
+
+class IDataType;
+using DataTypePtr = std::shared_ptr<const IDataType>;
+class MergeTreeTransaction;
+
+/// This macro is useful for places where a pointer to current transaction should be passed,
+/// but transactions are not supported yet (e.g. when calling MergeTreeData's methods from StorageReplicatedMergeTree)
+/// or transaction object is not needed and not passed intentionally.
+#ifndef NO_TRANSACTION_PTR
+#define NO_TRANSACTION_PTR std::shared_ptr<MergeTreeTransaction>(nullptr)
+#define NO_TRANSACTION_RAW static_cast<MergeTreeTransaction *>(nullptr)
+#endif
+
+/// Commit Sequence Number
+using CSN = UInt64;
+/// Local part of TransactionID
+using LocalTID = UInt64;
+/// Hash of TransactionID that fits into 64-bit atomic
+using TIDHash = UInt64;
+
+namespace Tx
+{
+    /// For transactions that are probably not committed (yet)
+    const CSN UnknownCSN = 0;
+    /// For changes were made without creating a transaction
+    const CSN PrehistoricCSN = 1;
+    /// Special reserved values
+    const CSN CommittingCSN = 2;
+    const CSN EverythingVisibleCSN = 3;
+    const CSN MaxReservedCSN = 32;
+
+    /// So far, that changes will never become visible
+    const CSN RolledBackCSN = std::numeric_limits<CSN>::max();
+
+    const LocalTID PrehistoricLocalTID = 1;
+    const LocalTID DummyLocalTID = 2;
+    const LocalTID MaxReservedLocalTID = 32;
+}
+
+struct TransactionID
+{
+    /// Global sequential number, the newest commit timestamp the we saw when this transaction began
+    CSN start_csn = 0;
+    /// Local sequential that is unique for each transaction started by this host within specific start_csn
+    LocalTID local_tid = 0;
+    /// UUID of host that has started this transaction
+    UUID host_id = UUIDHelpers::Nil;
+
+    /// NOTE Maybe we could just generate UUIDv4 for each transaction, but it would be harder to debug.
+    /// Partial order is defined for this TransactionID structure:
+    /// (tid1.start_csn <= tid2.start_csn)    <==>    (tid1 <= tid2)
+    /// (tid1.start_csn == tid2.start_csn && tid1.host_id == tid2.host_id && tid1.local_tid < tid2.local_tid)    ==>    (tid1 < tid2)
+    /// If two transaction have the same start_csn, but were started by different hosts, then order is undefined.
+
+    bool operator == (const TransactionID & rhs) const
+    {
+        return start_csn == rhs.start_csn && local_tid == rhs.local_tid && host_id == rhs.host_id;
+    }
+
+    bool operator != (const TransactionID & rhs) const
+    {
+        return !(*this == rhs);
+    }
+
+    TIDHash getHash() const;
+
+    bool isEmpty() const
+    {
+        assert((local_tid == 0) == (start_csn == 0 && host_id == UUIDHelpers::Nil));
+        return local_tid == 0;
+    }
+
+    bool isPrehistoric() const
+    {
+        assert((local_tid == Tx::PrehistoricLocalTID) == (start_csn == Tx::PrehistoricCSN));
+        return local_tid == Tx::PrehistoricLocalTID;
+    }
+
+
+    static void write(const TransactionID & tid, WriteBuffer & buf);
+    static TransactionID read(ReadBuffer & buf);
+};
+
+namespace Tx
+{
+    const TransactionID EmptyTID = {0, 0, UUIDHelpers::Nil};
+    const TransactionID PrehistoricTID = {PrehistoricCSN, PrehistoricLocalTID, UUIDHelpers::Nil};
+    const TransactionID DummyTID = {PrehistoricCSN, DummyLocalTID, UUIDHelpers::Nil};
+}
+
+}
+
+template<>
+struct fmt::formatter<DB::TransactionID>
+{
+    template<typename ParseContext>
+    constexpr auto parse(ParseContext & context)
+    {
+        return context.begin();
+    }
+
+    template<typename FormatContext>
+    auto format(const DB::TransactionID & tid, FormatContext & context)
+    {
+        return fmt::format_to(context.out(), "({}, {}, {})", tid.start_csn, tid.local_tid, tid.host_id);
+    }
+};

src/Common/ZooKeeper/ZooKeeper.cpp

@@ -1270,4 +1270,14 @@ String extractZooKeeperPath(const String & path, bool check_starts_with_slash, P
     return normalizeZooKeeperPath(path, check_starts_with_slash, log);
 }
 
+String getSequentialNodeName(const String & prefix, UInt64 number)
+{
+    /// NOTE Sequential counter in ZooKeeper is Int32.
+    assert(number < std::numeric_limits<Int32>::max());
+    constexpr size_t seq_node_digits = 10;
+    String num_str = std::to_string(number);
+    String name = prefix + String(seq_node_digits - num_str.size(), '0') + num_str;
+    return name;
+}
+
 }

src/Common/ZooKeeper/ZooKeeper.h

@@ -417,4 +417,6 @@ String extractZooKeeperName(const String & path);
 
 String extractZooKeeperPath(const String & path, bool check_starts_with_slash, Poco::Logger * log = nullptr);
 
+String getSequentialNodeName(const String & prefix, UInt64 number);
+
 }

src/Core/Settings.h

@@ -581,6 +581,7 @@ class IColumn;
     M(Bool, allow_experimental_nlp_functions, false, "Enable experimental functions for natural language processing.", 0) \
     M(Bool, allow_experimental_object_type, false, "Allow Object and JSON data types", 0) \
     M(String, insert_deduplication_token, "", "If not empty, used for duplicate detection instead of data digest", 0) \
+    M(Bool, throw_on_unsupported_query_inside_transaction, true, "Throw exception if unsupported query is used inside transaction", 0) \
 // End of COMMON_SETTINGS
 // Please add settings related to formats into the FORMAT_FACTORY_SETTINGS and move obsolete settings to OBSOLETE_SETTINGS.
 

src/Databases/DatabaseReplicated.cpp

@@ -461,6 +461,10 @@ void DatabaseReplicated::checkQueryValid(const ASTPtr & query, ContextPtr query_
 
 BlockIO DatabaseReplicated::tryEnqueueReplicatedDDL(const ASTPtr & query, ContextPtr query_context)
 {
+
+    if (query_context->getCurrentTransaction() && query_context->getSettingsRef().throw_on_unsupported_query_inside_transaction)
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Distributed DDL queries inside transactions are not supported");
+
     if (is_readonly)
         throw Exception(ErrorCodes::NO_ZOOKEEPER, "Database is in readonly mode, because it cannot connect to ZooKeeper");
 

src/Functions/FunctionConstantBase.h

@@ -41,9 +41,9 @@ public:
 
     bool isSuitableForShortCircuitArgumentsExecution(const DataTypesWithConstInfo & /*arguments*/) const override { return false; }
 
-    ColumnPtr executeImpl(const ColumnsWithTypeAndName &, const DataTypePtr &, size_t input_rows_count) const override
+    ColumnPtr executeImpl(const ColumnsWithTypeAndName &, const DataTypePtr & result_type, size_t input_rows_count) const override
     {
-        return ColumnT().createColumnConst(input_rows_count, constant_value);
+        return result_type->createColumnConst(input_rows_count, constant_value);
     }
 
 private:

src/Functions/FunctionsTransactionCounters.cpp

@@ -0,0 +1,71 @@
+#include <Functions/FunctionConstantBase.h>
+#include <DataTypes/DataTypeNothing.h>
+#include <DataTypes/DataTypesNumber.h>
+#include <Functions/FunctionFactory.h>
+#include <Interpreters/Context.h>
+#include <Interpreters/MergeTreeTransaction.h>
+#include <Interpreters/TransactionLog.h>
+
+
+namespace DB
+{
+
+namespace
+{
+
+class FunctionTransactionID : public FunctionConstantBase<FunctionTransactionID, Tuple, DataTypeNothing>
+{
+public:
+    static constexpr auto name = "transactionID";
+    static Tuple getValue(const MergeTreeTransactionPtr & txn)
+    {
+        Tuple res;
+        if (txn)
+            res = {txn->tid.start_csn, txn->tid.local_tid, txn->tid.host_id};
+        else
+            res = {UInt64(0), UInt64(0), UUIDHelpers::Nil};
+        return res;
+    }
+
+    DataTypePtr getReturnTypeImpl(const DataTypes & /*arguments*/) const override { return getTransactionIDDataType(); }
+
+    static FunctionPtr create(ContextPtr context) { return std::make_shared<FunctionTransactionID>(context); }
+    explicit FunctionTransactionID(ContextPtr context) : FunctionConstantBase(getValue(context->getCurrentTransaction()), context->isDistributed()) {}
+};
+
+class FunctionTransactionLatestSnapshot : public FunctionConstantBase<FunctionTransactionLatestSnapshot, UInt64, DataTypeUInt64>
+{
+    static UInt64 getLatestSnapshot(ContextPtr context)
+    {
+        context->checkTransactionsAreAllowed(/* explicit_tcl_query */ true);
+        return TransactionLog::instance().getLatestSnapshot();
+    }
+public:
+    static constexpr auto name = "transactionLatestSnapshot";
+    static FunctionPtr create(ContextPtr context) { return std::make_shared<FunctionTransactionLatestSnapshot>(context); }
+    explicit FunctionTransactionLatestSnapshot(ContextPtr context) : FunctionConstantBase(getLatestSnapshot(context), context->isDistributed()) {}
+};
+
+class FunctionTransactionOldestSnapshot : public FunctionConstantBase<FunctionTransactionOldestSnapshot, UInt64, DataTypeUInt64>
+{
+    static UInt64 getOldestSnapshot(ContextPtr context)
+    {
+        context->checkTransactionsAreAllowed(/* explicit_tcl_query */ true);
+        return TransactionLog::instance().getOldestSnapshot();
+    }
+public:
+    static constexpr auto name = "transactionOldestSnapshot";
+    static FunctionPtr create(ContextPtr context) { return std::make_shared<FunctionTransactionOldestSnapshot>(context); }
+    explicit FunctionTransactionOldestSnapshot(ContextPtr context) : FunctionConstantBase(getOldestSnapshot(context), context->isDistributed()) {}
+};
+
+}
+
+void registerFunctionsTransactionCounters(FunctionFactory & factory)
+{
+    factory.registerFunction<FunctionTransactionID>();
+    factory.registerFunction<FunctionTransactionLatestSnapshot>();
+    factory.registerFunction<FunctionTransactionOldestSnapshot>();
+}
+
+}

src/Functions/registerFunctionsMiscellaneous.cpp

@@ -75,6 +75,7 @@ void registerFunctionFile(FunctionFactory &);
 void registerFunctionConnectionId(FunctionFactory &);
 void registerFunctionPartitionId(FunctionFactory &);
 void registerFunctionIsIPAddressContainedIn(FunctionFactory &);
+void registerFunctionsTransactionCounters(FunctionFactory & factory);
 void registerFunctionQueryID(FunctionFactory &);
 void registerFunctionInitialQueryID(FunctionFactory &);
 void registerFunctionServerUUID(FunctionFactory &);
@@ -163,6 +164,7 @@ void registerFunctionsMiscellaneous(FunctionFactory & factory)
     registerFunctionConnectionId(factory);
     registerFunctionPartitionId(factory);
     registerFunctionIsIPAddressContainedIn(factory);
+    registerFunctionsTransactionCounters(factory);
     registerFunctionQueryID(factory);
     registerFunctionInitialQueryID(factory);
     registerFunctionServerUUID(factory);

src/IO/ReadHelpers.cpp

@@ -26,6 +26,7 @@ namespace ErrorCodes
     extern const int CANNOT_PARSE_DATETIME;
     extern const int CANNOT_PARSE_DATE;
     extern const int INCORRECT_DATA;
+    extern const int ATTEMPT_TO_READ_AFTER_EOF;
 }
 
 template <typename IteratorSrc, typename IteratorDst>
@@ -137,6 +138,12 @@ void assertEOF(ReadBuffer & buf)
         throwAtAssertionFailed("eof", buf);
 }
 
+void assertNotEOF(ReadBuffer & buf)
+{
+    if (buf.eof())
+        throw Exception("Attempt to read after EOF", ErrorCodes::ATTEMPT_TO_READ_AFTER_EOF);
+}
+
 
 void assertStringCaseInsensitive(const char * s, ReadBuffer & buf)
 {

src/IO/ReadHelpers.h

@@ -163,6 +163,7 @@ void readVectorBinary(std::vector<T> & v, ReadBuffer & buf, size_t MAX_VECTOR_SI
 
 void assertString(const char * s, ReadBuffer & buf);
 void assertEOF(ReadBuffer & buf);
+void assertNotEOF(ReadBuffer & buf);
 
 [[noreturn]] void throwAtAssertionFailed(const char * s, ReadBuffer & buf);
 

src/IO/WriteBufferFromFileDescriptor.cpp

@@ -133,7 +133,6 @@ off_t WriteBufferFromFileDescriptor::seek(off_t offset, int whence) // NOLINT
     return res;
 }
 
-
 void WriteBufferFromFileDescriptor::truncate(off_t length) // NOLINT
 {
     int res = ftruncate(fd, length);

src/IO/WriteHelpers.h

@@ -1163,3 +1163,19 @@ struct PcgSerializer
 void writePointerHex(const void * ptr, WriteBuffer & buf);
 
 }
+
+template<>
+struct fmt::formatter<DB::UUID>
+{
+    template<typename ParseContext>
+    constexpr auto parse(ParseContext & context)
+    {
+        return context.begin();
+    }
+
+    template<typename FormatContext>
+    auto format(const DB::UUID & uuid, FormatContext & context)
+    {
+        return fmt::format_to(context.out(), "{}", toString(uuid));
+    }
+};

src/Interpreters/Context.cpp

@@ -90,6 +90,7 @@
 #include <Interpreters/SynonymsExtensions.h>
 #include <Interpreters/Lemmatizers.h>
 #include <Interpreters/ClusterDiscovery.h>
+#include <Interpreters/TransactionLog.h>
 #include <filesystem>
 
 #if USE_ROCKSDB
@@ -133,6 +134,7 @@ namespace ErrorCodes
     extern const int LOGICAL_ERROR;
     extern const int INVALID_SETTING_VALUE;
     extern const int UNKNOWN_READ_METHOD;
+    extern const int NOT_IMPLEMENTED;
 }
 
 
@@ -281,6 +283,8 @@ struct ContextSharedPart
 
     Context::ConfigReloadCallback config_reload_callback;
 
+    bool is_server_completely_started = false;
+
 #if USE_ROCKSDB
     /// Global merge tree metadata cache, stored in rocksdb.
     MergeTreeMetadataCachePtr merge_tree_metadata_cache;
@@ -365,6 +369,8 @@ struct ContextSharedPart
         if (common_executor)
             common_executor->wait();
 
+        TransactionLog::shutdownIfAny();
+
         std::unique_ptr<SystemLogs> delete_system_logs;
         std::unique_ptr<EmbeddedDictionaries> delete_embedded_dictionaries;
         std::unique_ptr<ExternalDictionariesLoader> delete_external_dictionaries_loader;
@@ -492,6 +498,8 @@ ContextMutablePtr Context::createGlobal(ContextSharedPart * shared)
 
 void Context::initGlobal()
 {
+    assert(!global_context_instance);
+    global_context_instance = shared_from_this();
     DatabaseCatalog::init(shared_from_this());
 }
 
@@ -2475,6 +2483,17 @@ std::shared_ptr<ZooKeeperLog> Context::getZooKeeperLog() const
 }
 
 
+std::shared_ptr<TransactionsInfoLog> Context::getTransactionsInfoLog() const
+{
+    auto lock = getLock();
+
+    if (!shared->system_logs)
+        return {};
+
+    return shared->system_logs->transactions_info_log;
+}
+
+
 std::shared_ptr<ProcessorsProfileLog> Context::getProcessorsProfileLog() const
 {
     auto lock = getLock();
@@ -3078,6 +3097,56 @@ void Context::resetZooKeeperMetadataTransaction()
     metadata_transaction = nullptr;
 }
 
+
+void Context::checkTransactionsAreAllowed(bool explicit_tcl_query /* = false */) const
+{
+    if (getConfigRef().getInt("allow_experimental_transactions", 0))
+        return;
+
+    if (explicit_tcl_query)
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Transactions are not supported");
+
+    throw Exception(ErrorCodes::LOGICAL_ERROR, "Experimental support for transactions is disabled, "
+                    "however, some query or background task tried to access TransactionLog. "
+                    "If you have not enabled this feature explicitly, then it's a bug.");
+}
+
+void Context::initCurrentTransaction(MergeTreeTransactionPtr txn)
+{
+    merge_tree_transaction_holder = MergeTreeTransactionHolder(txn, false, this);
+    setCurrentTransaction(std::move(txn));
+}
+
+void Context::setCurrentTransaction(MergeTreeTransactionPtr txn)
+{
+    assert(!merge_tree_transaction || !txn);
+    assert(this == session_context.lock().get() || this == query_context.lock().get());
+    merge_tree_transaction = std::move(txn);
+    if (!merge_tree_transaction)
+        merge_tree_transaction_holder = {};
+}
+
+MergeTreeTransactionPtr Context::getCurrentTransaction() const
+{
+    return merge_tree_transaction;
+}
+
+bool Context::isServerCompletelyStarted() const
+{
+    auto lock = getLock();
+    assert(getApplicationType() == ApplicationType::SERVER);
+    return shared->is_server_completely_started;
+}
+
+void Context::setServerCompletelyStarted()
+{
+    auto lock = getLock();
+    assert(global_context.lock().get() == this);
+    assert(!shared->is_server_completely_started);
+    assert(getApplicationType() == ApplicationType::SERVER);
+    shared->is_server_completely_started = true;
+}
+
 PartUUIDsPtr Context::getPartUUIDs() const
 {
     auto lock = getLock();

src/Interpreters/Context.h

@@ -7,6 +7,7 @@
 #include <Interpreters/ClientInfo.h>
 #include <Interpreters/Context_fwd.h>
 #include <Interpreters/DatabaseCatalog.h>
+#include <Interpreters/MergeTreeTransactionHolder.h>
 #include <Parsers/IAST_fwd.h>
 #include <Storages/IStorage_fwd.h>
 #include <Common/MultiVersion.h>
@@ -25,6 +26,8 @@
 #include <memory>
 #include <mutex>
 #include <optional>
+
+#include <thread>
 #include <exception>
 
 
@@ -80,6 +83,7 @@ class AsynchronousMetricLog;
 class OpenTelemetrySpanLog;
 class ZooKeeperLog;
 class SessionLog;
+class TransactionsInfoLog;
 class ProcessorsProfileLog;
 struct MergeTreeSettings;
 class StorageS3Settings;
@@ -313,6 +317,7 @@ private:
     /// A flag, used to distinguish between user query and internal query to a database engine (MaterializedPostgreSQL).
     bool is_internal_query = false;
 
+    inline static ContextPtr global_context_instance;
 
 public:
     // Top-level OpenTelemetry trace context for the query. Makes sense only for a query context.
@@ -340,6 +345,11 @@ private:
                                                     /// thousands of signatures.
                                                     /// And I hope it will be replaced with more common Transaction sometime.
 
+    MergeTreeTransactionPtr merge_tree_transaction;     /// Current transaction context. Can be inside session or query context.
+                                                        /// It's shared with all children contexts.
+    MergeTreeTransactionHolder merge_tree_transaction_holder;   /// It will rollback or commit transaction on Context destruction.
+
+    /// Use copy constructor or createGlobal() instead
     Context();
     Context(const Context &);
     Context & operator=(const Context &);
@@ -634,6 +644,8 @@ public:
 
     ContextMutablePtr getGlobalContext() const;
 
+    static ContextPtr getGlobalContextInstance() { return global_context_instance; }
+
     bool hasGlobalContext() const { return !global_context.expired(); }
     bool isGlobalContext() const
     {
@@ -803,6 +815,7 @@ public:
     std::shared_ptr<OpenTelemetrySpanLog> getOpenTelemetrySpanLog() const;
     std::shared_ptr<ZooKeeperLog> getZooKeeperLog() const;
     std::shared_ptr<SessionLog> getSessionLog() const;
+    std::shared_ptr<TransactionsInfoLog> getTransactionsInfoLog() const;
     std::shared_ptr<ProcessorsProfileLog> getProcessorsProfileLog() const;
 
     /// Returns an object used to log operations with parts if it possible.
@@ -891,6 +904,14 @@ public:
     /// Removes context of current distributed DDL.
     void resetZooKeeperMetadataTransaction();
 
+    void checkTransactionsAreAllowed(bool explicit_tcl_query = false) const;
+    void initCurrentTransaction(MergeTreeTransactionPtr txn);
+    void setCurrentTransaction(MergeTreeTransactionPtr txn);
+    MergeTreeTransactionPtr getCurrentTransaction() const;
+
+    bool isServerCompletelyStarted() const;
+    void setServerCompletelyStarted();
+
     PartUUIDsPtr getPartUUIDs() const;
     PartUUIDsPtr getIgnoredPartUUIDs() const;
 

src/Interpreters/DDLTask.cpp

@@ -391,12 +391,7 @@ ContextMutablePtr DatabaseReplicatedTask::makeQueryContext(ContextPtr from_conte
 
 String DDLTaskBase::getLogEntryName(UInt32 log_entry_number)
 {
-    /// Sequential counter in ZooKeeper is Int32.
-    assert(log_entry_number < std::numeric_limits<Int32>::max());
-    constexpr size_t seq_node_digits = 10;
-    String number = toString(log_entry_number);
-    String name = "query-" + String(seq_node_digits - number.size(), '0') + number;
-    return name;
+    return zkutil::getSequentialNodeName("query-", log_entry_number);
 }
 
 UInt32 DDLTaskBase::getLogEntryNumber(const String & log_entry_name)

src/Interpreters/IInterpreter.cpp

@@ -1,9 +1,16 @@
 #include <Interpreters/IInterpreter.h>
 #include <Interpreters/QueryLog.h>
 #include <Interpreters/Context.h>
+#include <Storages/IStorage.h>
 
 namespace DB
 {
+
+namespace ErrorCodes
+{
+    extern const int NOT_IMPLEMENTED;
+}
+
 void IInterpreter::extendQueryLogElem(
     QueryLogElement & elem, const ASTPtr & ast, ContextPtr context, const String & query_database, const String & query_table) const
 {
@@ -21,4 +28,18 @@ void IInterpreter::extendQueryLogElem(
 
     extendQueryLogElemImpl(elem, ast, context);
 }
+
+void IInterpreter::checkStorageSupportsTransactionsIfNeeded(const StoragePtr & storage, ContextPtr context)
+{
+    if (!context->getCurrentTransaction())
+        return;
+
+    if (storage->supportsTransactions())
+        return;
+
+    if (context->getSettingsRef().throw_on_unsupported_query_inside_transaction)
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Storage {} (table {}) does not support transactions",
+                        storage->getName(), storage->getStorageID().getNameForLogs());
+}
+
 }

src/Interpreters/IInterpreter.h

@@ -3,6 +3,7 @@
 #include <QueryPipeline/BlockIO.h>
 #include <Interpreters/Context_fwd.h>
 #include <Parsers/IAST_fwd.h>
+#include <Storages//IStorage_fwd.h>
 
 namespace DB
 {
@@ -33,6 +34,13 @@ public:
 
     virtual void extendQueryLogElemImpl(QueryLogElement &, const ASTPtr &, ContextPtr) const {}
 
+    /// Returns true if transactions maybe supported for this type of query.
+    /// If Interpreter returns true, than it is responsible to check that specific query with specific Storage is supported.
+    virtual bool supportsTransactions() const { return false; }
+
+    /// Helper function for some Interpreters.
+    static void checkStorageSupportsTransactionsIfNeeded(const StoragePtr & storage, ContextPtr context);
+
     virtual ~IInterpreter() = default;
 };
 

src/Interpreters/InterpreterAlterQuery.cpp

@@ -86,6 +86,7 @@ BlockIO InterpreterAlterQuery::executeToTable(const ASTAlterQuery & alter)
     }
 
     StoragePtr table = DatabaseCatalog::instance().getTable(table_id, getContext());
+    checkStorageSupportsTransactionsIfNeeded(table, getContext());
     if (table->isStaticStorage())
         throw Exception(ErrorCodes::TABLE_IS_READ_ONLY, "Table is read-only");
     auto table_lock = table->lockForShare(getContext()->getCurrentQueryId(), getContext()->getSettingsRef().lock_acquire_timeout);

src/Interpreters/InterpreterAlterQuery.h

@@ -26,6 +26,8 @@ public:
 
     void extendQueryLogElemImpl(QueryLogElement & elem, const ASTPtr & ast, ContextPtr context) const override;
 
+    bool supportsTransactions() const override { return true; }
+
 private:
     AccessRightsElements getRequiredAccess() const;
 

src/Interpreters/InterpreterDescribeQuery.cpp

@@ -89,7 +89,7 @@ BlockIO InterpreterDescribeQuery::execute()
         auto table_lock = table->lockForShare(getContext()->getInitialQueryId(), settings.lock_acquire_timeout);
 
         auto metadata_snapshot = table->getInMemoryMetadataPtr();
-        storage_snapshot = table->getStorageSnapshot(metadata_snapshot);
+        storage_snapshot = table->getStorageSnapshot(metadata_snapshot, getContext());
         columns = metadata_snapshot->getColumns();
     }
 

src/Interpreters/InterpreterExplainQuery.cpp

@@ -9,6 +9,7 @@
 #include <Interpreters/InterpreterInsertQuery.h>
 #include <Interpreters/Context.h>
 #include <Interpreters/TableOverrideUtils.h>
+#include <Interpreters/MergeTreeTransaction.h>
 #include <Formats/FormatFactory.h>
 #include <Parsers/DumpASTNode.h>
 #include <Parsers/queryToString.h>
@@ -400,6 +401,23 @@ QueryPipeline InterpreterExplainQuery::executeImpl()
             override_info.appendTo(buf);
             break;
         }
+        case ASTExplainQuery::CurrentTransaction:
+        {
+            if (ast.getSettings())
+                throw Exception("Settings are not supported for EXPLAIN CURRENT TRANSACTION query.", ErrorCodes::UNKNOWN_SETTING);
+
+            if (auto txn = getContext()->getCurrentTransaction())
+            {
+                String dump = txn->dumpDescription();
+                buf.write(dump.data(), dump.size());
+            }
+            else
+            {
+                writeCString("<no current transaction>", buf);
+            }
+
+            break;
+        }
     }
     if (insert_buf)
     {

src/Interpreters/InterpreterExplainQuery.h

@@ -17,6 +17,8 @@ public:
 
     static Block getSampleBlock(ASTExplainQuery::ExplainKind kind);
 
+    bool supportsTransactions() const override { return true; }
+
 private:
     ASTPtr query;
 

src/Interpreters/InterpreterFactory.cpp

@@ -19,6 +19,7 @@
 #include <Parsers/ASTUseQuery.h>
 #include <Parsers/ASTWatchQuery.h>
 #include <Parsers/MySQL/ASTCreateQuery.h>
+#include <Parsers/ASTTransactionControl.h>
 #include <Parsers/TablePropertiesQueriesASTs.h>
 
 #include <Parsers/Access/ASTCreateQuotaQuery.h>
@@ -62,6 +63,7 @@
 #include <Interpreters/InterpreterSystemQuery.h>
 #include <Interpreters/InterpreterUseQuery.h>
 #include <Interpreters/InterpreterWatchQuery.h>
+#include <Interpreters/InterpreterTransactionControlQuery.h>
 #include <Interpreters/OpenTelemetrySpanLog.h>
 
 #include <Interpreters/Access/InterpreterCreateQuotaQuery.h>
@@ -278,6 +280,10 @@ std::unique_ptr<IInterpreter> InterpreterFactory::get(ASTPtr & query, ContextMut
     {
         return std::make_unique<InterpreterExternalDDLQuery>(query, context);
     }
+    else if (query->as<ASTTransactionControl>())
+    {
+        return std::make_unique<InterpreterTransactionControlQuery>(query, context);
+    }
     else if (query->as<ASTCreateFunctionQuery>())
     {
         return std::make_unique<InterpreterCreateFunctionQuery>(query, context, true /*persist_function*/);

src/Interpreters/InterpreterInsertQuery.cpp

@@ -287,6 +287,8 @@ BlockIO InterpreterInsertQuery::execute()
     QueryPipelineBuilder pipeline;
 
     StoragePtr table = getTable(query);
+    checkStorageSupportsTransactionsIfNeeded(table, getContext());
+
     StoragePtr inner_table;
     if (const auto * mv = dynamic_cast<const StorageMaterializedView *>(table.get()))
         inner_table = mv->getTargetTable();

src/Interpreters/InterpreterInsertQuery.h

@@ -46,6 +46,8 @@ public:
     StoragePtr getTable(ASTInsertQuery & query);
     Block getSampleBlock(const ASTInsertQuery & query, const StoragePtr & table, const StorageMetadataPtr & metadata_snapshot) const;
 
+    bool supportsTransactions() const override { return true; }
+
 private:
     Block getSampleBlock(const Names & names, const StoragePtr & table, const StorageMetadataPtr & metadata_snapshot) const;
 

src/Interpreters/InterpreterKillQueryQuery.cpp

@@ -7,6 +7,7 @@
 #include <Interpreters/executeQuery.h>
 #include <Interpreters/CancellationCode.h>
 #include <Interpreters/InterpreterAlterQuery.h>
+#include <Interpreters/TransactionLog.h>
 #include <Parsers/ASTAlterQuery.h>
 #include <Parsers/ParserAlterQuery.h>
 #include <Parsers/parseQuery.h>
@@ -358,6 +359,49 @@ BlockIO InterpreterKillQueryQuery::execute()
 
         break;
     }
+    case ASTKillQueryQuery::Type::Transaction:
+    {
+        getContext()->checkAccess(AccessType::KILL_TRANSACTION);
+
+        Block transactions_block = getSelectResult("tid, tid_hash, elapsed, is_readonly, state", "system.transactions");
+
+        if (!transactions_block)
+            return res_io;
+
+        const ColumnUInt64 & tid_hash_col = typeid_cast<const ColumnUInt64 &>(*transactions_block.getByName("tid_hash").column);
+
+        auto header = transactions_block.cloneEmpty();
+        header.insert(0, {ColumnString::create(), std::make_shared<DataTypeString>(), "kill_status"});
+        MutableColumns res_columns = header.cloneEmptyColumns();
+
+        for (size_t i = 0; i < transactions_block.rows(); ++i)
+        {
+            UInt64 tid_hash = tid_hash_col.getUInt(i);
+
+            CancellationCode code = CancellationCode::Unknown;
+            if (!query.test)
+            {
+                auto txn = TransactionLog::instance().tryGetRunningTransaction(tid_hash);
+                if (txn)
+                {
+                    txn->onException();
+                    if (txn->getState() == MergeTreeTransaction::ROLLED_BACK)
+                        code = CancellationCode::CancelSent;
+                    else
+                        code = CancellationCode::CancelCannotBeSent;
+                }
+                else
+                {
+                    code = CancellationCode::NotFound;
+                }
+            }
+
+            insertResultRow(i, code, transactions_block, header, res_columns);
+        }
+
+        res_io.pipeline = QueryPipeline(Pipe(std::make_shared<SourceFromSingleChunk>(header.cloneWithColumns(std::move(res_columns)))));
+        break;
+    }
     }
 
     return res_io;

src/Interpreters/InterpreterOptimizeQuery.cpp

@@ -31,8 +31,9 @@ BlockIO InterpreterOptimizeQuery::execute()
 
     auto table_id = getContext()->resolveStorageID(ast, Context::ResolveOrdinary);
     StoragePtr table = DatabaseCatalog::instance().getTable(table_id, getContext());
+    checkStorageSupportsTransactionsIfNeeded(table, getContext());
     auto metadata_snapshot = table->getInMemoryMetadataPtr();
-    auto storage_snapshot = table->getStorageSnapshot(metadata_snapshot);
+    auto storage_snapshot = table->getStorageSnapshot(metadata_snapshot, getContext());
 
     // Empty list of names means we deduplicate by all columns, but user can explicitly state which columns to use.
     Names column_names;

src/Interpreters/InterpreterOptimizeQuery.h

@@ -18,6 +18,8 @@ public:
 
     BlockIO execute() override;
 
+    bool supportsTransactions() const override { return true; }
+
 private:
     AccessRightsElements getRequiredAccess() const;
 

src/Interpreters/InterpreterSelectQuery.cpp

@@ -329,12 +329,28 @@ InterpreterSelectQuery::InterpreterSelectQuery(
         if (!metadata_snapshot)
             metadata_snapshot = storage->getInMemoryMetadataPtr();
 
-        storage_snapshot = storage->getStorageSnapshotForQuery(metadata_snapshot, query_ptr);
+        storage_snapshot = storage->getStorageSnapshotForQuery(metadata_snapshot, query_ptr, context);
     }
 
     if (has_input || !joined_tables.resolveTables())
         joined_tables.makeFakeTable(storage, metadata_snapshot, source_header);
 
+
+    if (context->getCurrentTransaction() && context->getSettingsRef().throw_on_unsupported_query_inside_transaction)
+    {
+        if (storage)
+            checkStorageSupportsTransactionsIfNeeded(storage, context);
+        for (const auto & table : joined_tables.tablesWithColumns())
+        {
+            if (table.table.table.empty())
+                continue;
+            auto maybe_storage = DatabaseCatalog::instance().tryGetTable({table.table.database, table.table.table}, context);
+            if (!maybe_storage)
+                continue;
+            checkStorageSupportsTransactionsIfNeeded(storage, context);
+        }
+    }
+
     /// Rewrite JOINs
     if (!has_input && joined_tables.tablesCount() > 1)
     {
@@ -1791,7 +1807,7 @@ void InterpreterSelectQuery::executeFetchColumns(QueryProcessingStage::Enum proc
         const auto & func = desc.function;
         std::optional<UInt64> num_rows{};
 
-        if (!query.prewhere() && !query.where())
+        if (!query.prewhere() && !query.where() && !context->getCurrentTransaction())
         {
             num_rows = storage->totalRows(settings);
         }

src/Interpreters/InterpreterSelectQuery.h

@@ -104,6 +104,8 @@ public:
 
     Names getRequiredColumns() { return required_columns; }
 
+    bool supportsTransactions() const override { return true; }
+
 private:
     InterpreterSelectQuery(
         const ASTPtr & query_ptr_,

src/Interpreters/InterpreterSelectWithUnionQuery.h

@@ -39,6 +39,8 @@ public:
 
     virtual void ignoreWithTotals() override;
 
+    bool supportsTransactions() const override { return true; }
+
 private:
     std::vector<std::unique_ptr<IInterpreterUnionOrSelectQuery>> nested_interpreters;
 

src/Interpreters/InterpreterSetQuery.h

@@ -25,6 +25,8 @@ public:
       */
     void executeForCurrentContext();
 
+    bool supportsTransactions() const override { return true; }
+
 private:
     ASTPtr query_ptr;
 };

src/Interpreters/InterpreterSystemQuery.cpp

@@ -31,6 +31,7 @@
 #include <Interpreters/AsynchronousMetricLog.h>
 #include <Interpreters/OpenTelemetrySpanLog.h>
 #include <Interpreters/ZooKeeperLog.h>
+#include <Interpreters/TransactionsInfoLog.h>
 #include <Interpreters/ProcessorsProfileLog.h>
 #include <Interpreters/JIT/CompiledExpressionCache.h>
 #include <Access/ContextAccess.h>
@@ -462,6 +463,7 @@ BlockIO InterpreterSystemQuery::execute()
                 [&] { if (auto query_views_log = getContext()->getQueryViewsLog()) query_views_log->flush(true); },
                 [&] { if (auto zookeeper_log = getContext()->getZooKeeperLog()) zookeeper_log->flush(true); },
                 [&] { if (auto session_log = getContext()->getSessionLog()) session_log->flush(true); },
+                [&] { if (auto transactions_info_log = getContext()->getTransactionsInfoLog()) transactions_info_log->flush(true); },
                 [&] { if (auto processors_profile_log = getContext()->getProcessorsProfileLog()) processors_profile_log->flush(true); }
             );
             break;

src/Interpreters/InterpreterTransactionControlQuery.cpp

@@ -0,0 +1,90 @@
+#include <Interpreters/InterpreterTransactionControlQuery.h>
+#include <Parsers/ASTTransactionControl.h>
+#include <Interpreters/TransactionLog.h>
+#include <Interpreters/Context.h>
+
+namespace DB
+{
+
+namespace ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+    extern const int INVALID_TRANSACTION;
+}
+
+BlockIO InterpreterTransactionControlQuery::execute()
+{
+    if (!query_context->hasSessionContext())
+        throw Exception(ErrorCodes::INVALID_TRANSACTION, "Transaction Control Language queries are allowed only inside session");
+
+    ContextMutablePtr session_context = query_context->getSessionContext();
+    const auto & tcl = query_ptr->as<const ASTTransactionControl &>();
+
+    switch (tcl.action)
+    {
+        case ASTTransactionControl::BEGIN:
+            return executeBegin(session_context);
+        case ASTTransactionControl::COMMIT:
+            return executeCommit(session_context);
+        case ASTTransactionControl::ROLLBACK:
+            return executeRollback(session_context);
+        case ASTTransactionControl::SET_SNAPSHOT:
+            return executeSetSnapshot(session_context, tcl.snapshot);
+    }
+    assert(false);
+    __builtin_unreachable();
+}
+
+BlockIO InterpreterTransactionControlQuery::executeBegin(ContextMutablePtr session_context)
+{
+    if (session_context->getCurrentTransaction())
+        throw Exception(ErrorCodes::INVALID_TRANSACTION, "Nested transactions are not supported");
+
+    session_context->checkTransactionsAreAllowed(/* explicit_tcl_query = */ true);
+    auto txn = TransactionLog::instance().beginTransaction();
+    session_context->initCurrentTransaction(txn);
+    query_context->setCurrentTransaction(txn);
+    return {};
+}
+
+BlockIO InterpreterTransactionControlQuery::executeCommit(ContextMutablePtr session_context)
+{
+    auto txn = session_context->getCurrentTransaction();
+    if (!txn)
+        throw Exception(ErrorCodes::INVALID_TRANSACTION, "There is no current transaction");
+    if (txn->getState() != MergeTreeTransaction::RUNNING)
+        throw Exception(ErrorCodes::INVALID_TRANSACTION, "Transaction is not in RUNNING state");
+
+    TransactionLog::instance().commitTransaction(txn);
+    session_context->setCurrentTransaction(NO_TRANSACTION_PTR);
+    return {};
+}
+
+BlockIO InterpreterTransactionControlQuery::executeRollback(ContextMutablePtr session_context)
+{
+    auto txn = session_context->getCurrentTransaction();
+    if (!txn)
+        throw Exception(ErrorCodes::INVALID_TRANSACTION, "There is no current transaction");
+    if (txn->getState() == MergeTreeTransaction::COMMITTED)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "Transaction is in COMMITTED state");
+
+    if (txn->getState() == MergeTreeTransaction::RUNNING)
+        TransactionLog::instance().rollbackTransaction(txn);
+    session_context->setCurrentTransaction(NO_TRANSACTION_PTR);
+    return {};
+}
+
+BlockIO InterpreterTransactionControlQuery::executeSetSnapshot(ContextMutablePtr session_context, UInt64 snapshot)
+{
+    auto txn = session_context->getCurrentTransaction();
+    if (!txn)
+        throw Exception(ErrorCodes::INVALID_TRANSACTION, "There is no current transaction");
+
+    if (snapshot <= Tx::MaxReservedCSN && snapshot != Tx::PrehistoricCSN && snapshot != Tx::EverythingVisibleCSN)
+        throw Exception(ErrorCodes::INVALID_TRANSACTION, "Cannot set snapshot to reserved CSN");
+
+    txn->setSnapshot(snapshot);
+    return {};
+}
+
+}

src/Interpreters/InterpreterTransactionControlQuery.h

@@ -0,0 +1,34 @@
+#pragma once
+#include <Interpreters/IInterpreter.h>
+#include <Parsers/IAST_fwd.h>
+
+namespace DB
+{
+
+class InterpreterTransactionControlQuery : public IInterpreter
+{
+public:
+    InterpreterTransactionControlQuery(const ASTPtr & query_ptr_, ContextMutablePtr context_)
+    : query_context(context_)
+    , query_ptr(query_ptr_)
+    {
+    }
+
+    BlockIO execute() override;
+
+    bool ignoreQuota() const override { return true; }
+    bool ignoreLimits() const override { return true; }
+    bool supportsTransactions() const override { return true; }
+
+private:
+    BlockIO executeBegin(ContextMutablePtr session_context);
+    static BlockIO executeCommit(ContextMutablePtr session_context);
+    static BlockIO executeRollback(ContextMutablePtr session_context);
+    static BlockIO executeSetSnapshot(ContextMutablePtr session_context, UInt64 snapshot);
+
+private:
+    ContextMutablePtr query_context;
+    ASTPtr query_ptr;
+};
+
+}

src/Interpreters/MergeTreeTransaction.cpp

@@ -0,0 +1,351 @@
+#include <Interpreters/MergeTreeTransaction.h>
+#include <Storages/MergeTree/IMergeTreeDataPart.h>
+#include <Storages/MergeTree/MergeTreeData.h>
+#include <Interpreters/TransactionLog.h>
+#include <Interpreters/TransactionsInfoLog.h>
+
+namespace DB
+{
+
+namespace ErrorCodes
+{
+    extern const int INVALID_TRANSACTION;
+    extern const int LOGICAL_ERROR;
+}
+
+static TableLockHolder getLockForOrdinary(const StoragePtr & storage)
+{
+    if (storage->getStorageID().uuid != UUIDHelpers::Nil)
+        return {};
+
+    /// Maybe we should just throw an exception and do not support Ordinary database?
+    auto default_timeout = std::chrono::milliseconds(10 * 1000);
+    return storage->lockForShare(RWLockImpl::NO_QUERY, default_timeout);
+}
+
+MergeTreeTransaction::MergeTreeTransaction(CSN snapshot_, LocalTID local_tid_, UUID host_id)
+    : tid({snapshot_, local_tid_, host_id})
+    , snapshot(snapshot_)
+    , csn(Tx::UnknownCSN)
+{
+}
+
+void MergeTreeTransaction::setSnapshot(CSN new_snapshot)
+{
+    snapshot = new_snapshot;
+}
+
+MergeTreeTransaction::State MergeTreeTransaction::getState() const
+{
+    CSN c = csn.load();
+    if (c == Tx::UnknownCSN || c == Tx::CommittingCSN)
+        return RUNNING;
+    if (c == Tx::RolledBackCSN)
+        return ROLLED_BACK;
+    return COMMITTED;
+}
+
+void MergeTreeTransaction::checkIsNotCancelled() const
+{
+    CSN c = csn.load();
+    if (c == Tx::RolledBackCSN)
+        throw Exception(ErrorCodes::INVALID_TRANSACTION, "Transaction was cancelled");
+    else if (c != Tx::UnknownCSN)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "Unexpected CSN state: {}", c);
+}
+
+void MergeTreeTransaction::addNewPart(const StoragePtr & storage, const DataPartPtr & new_part, MergeTreeTransaction * txn)
+{
+    /// Creation TID was written to data part earlier on part creation.
+    /// We only need to ensure that it's written and add part to in-memory set of new parts.
+    new_part->assertHasVersionMetadata(txn);
+    if (txn)
+    {
+        txn->addNewPart(storage, new_part);
+        /// Now we know actual part name and can write it to system log table.
+        tryWriteEventToSystemLog(new_part->version.log, TransactionsInfoLogElement::ADD_PART, txn->tid, TransactionInfoContext{storage->getStorageID(), new_part->name});
+    }
+}
+
+void MergeTreeTransaction::removeOldPart(const StoragePtr & storage, const DataPartPtr & part_to_remove, MergeTreeTransaction * txn)
+{
+    TransactionInfoContext transaction_context{storage->getStorageID(), part_to_remove->name};
+    if (txn)
+    {
+        /// Lock part for removal and write current TID into version metadata file.
+        /// If server crash just after committing transactions
+        /// we will find this TID in version metadata and will finally remove part.
+        txn->removeOldPart(storage, part_to_remove, transaction_context);
+    }
+    else
+    {
+        /// Lock part for removal with special TID, so transactions will not try to remove it concurrently.
+        /// We lock it only in memory if part was not involved in any transactions.
+        part_to_remove->version.lockRemovalTID(Tx::PrehistoricTID, transaction_context);
+        if (part_to_remove->wasInvolvedInTransaction())
+            part_to_remove->appendRemovalTIDToVersionMetadata();
+    }
+}
+
+void MergeTreeTransaction::addNewPartAndRemoveCovered(const StoragePtr & storage, const DataPartPtr & new_part, const DataPartsVector & covered_parts, MergeTreeTransaction * txn)
+{
+    TransactionID tid = txn ? txn->tid : Tx::PrehistoricTID;
+    TransactionInfoContext transaction_context{storage->getStorageID(), new_part->name};
+    tryWriteEventToSystemLog(new_part->version.log, TransactionsInfoLogElement::ADD_PART, tid, transaction_context);
+    transaction_context.covering_part = std::move(transaction_context.part_name);
+    new_part->assertHasVersionMetadata(txn);
+
+    if (txn)
+    {
+        txn->addNewPart(storage, new_part);
+        for (const auto & covered : covered_parts)
+        {
+            transaction_context.part_name = covered->name;
+            txn->removeOldPart(storage, covered, transaction_context);
+        }
+    }
+    else
+    {
+        for (const auto & covered : covered_parts)
+        {
+            transaction_context.part_name = covered->name;
+            covered->version.lockRemovalTID(tid, transaction_context);
+        }
+    }
+}
+
+void MergeTreeTransaction::addNewPart(const StoragePtr & storage, const DataPartPtr & new_part)
+{
+    auto maybe_lock = getLockForOrdinary(storage);
+    std::lock_guard lock{mutex};
+    checkIsNotCancelled();
+    storages.insert(storage);
+    if (maybe_lock)
+        table_read_locks_for_ordinary_db.emplace_back(std::move(maybe_lock));
+    creating_parts.push_back(new_part);
+}
+
+void MergeTreeTransaction::removeOldPart(const StoragePtr & storage, const DataPartPtr & part_to_remove, const TransactionInfoContext & context)
+{
+    auto maybe_lock = getLockForOrdinary(storage);
+
+    {
+        std::lock_guard lock{mutex};
+        checkIsNotCancelled();
+
+        LockMemoryExceptionInThread lock_memory_tracker(VariableContext::Global);
+        part_to_remove->version.lockRemovalTID(tid, context);
+        storages.insert(storage);
+        if (maybe_lock)
+            table_read_locks_for_ordinary_db.emplace_back(std::move(maybe_lock));
+        removing_parts.push_back(part_to_remove);
+    }
+
+    part_to_remove->appendRemovalTIDToVersionMetadata();
+}
+
+void MergeTreeTransaction::addMutation(const StoragePtr & table, const String & mutation_id)
+{
+    auto maybe_lock = getLockForOrdinary(table);
+    std::lock_guard lock{mutex};
+    checkIsNotCancelled();
+    storages.insert(table);
+    if (maybe_lock)
+        table_read_locks_for_ordinary_db.emplace_back(std::move(maybe_lock));
+    mutations.emplace_back(table, mutation_id);
+}
+
+bool MergeTreeTransaction::isReadOnly() const
+{
+    std::lock_guard lock{mutex};
+    assert((creating_parts.empty() && removing_parts.empty() && mutations.empty()) == storages.empty());
+    return storages.empty();
+}
+
+scope_guard MergeTreeTransaction::beforeCommit()
+{
+    RunningMutationsList mutations_to_wait;
+    {
+        std::lock_guard lock{mutex};
+        mutations_to_wait = mutations;
+    }
+
+    /// We should wait for mutations to finish before committing transaction, because some mutation may fail and cause rollback.
+    for (const auto & table_and_mutation : mutations_to_wait)
+        table_and_mutation.first->waitForMutation(table_and_mutation.second);
+
+    assert([&]()
+    {
+        std::lock_guard lock{mutex};
+        return mutations == mutations_to_wait;
+    }());
+
+    CSN expected = Tx::UnknownCSN;
+    bool can_commit = csn.compare_exchange_strong(expected, Tx::CommittingCSN);
+    if (!can_commit)
+    {
+        /// Transaction was concurrently cancelled by KILL TRANSACTION or KILL MUTATION
+        if (expected == Tx::RolledBackCSN)
+            throw Exception(ErrorCodes::INVALID_TRANSACTION, "Transaction was cancelled");
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "Unexpected CSN state: {}", expected);
+    }
+
+    /// We should set CSN back to Unknown if we will fail to commit transaction for some reason (connection loss, etc)
+    return [this]()
+    {
+        CSN expected_value = Tx::CommittingCSN;
+        csn.compare_exchange_strong(expected_value, Tx::UnknownCSN);
+    };
+}
+
+void MergeTreeTransaction::afterCommit(CSN assigned_csn) noexcept
+{
+    /// Write allocated CSN into version metadata, so we will know CSN without reading it from transaction log
+    /// and we will be able to remove old entries from transaction log in ZK.
+    /// It's not a problem if server crash before CSN is written, because we already have TID in data part and entry in the log.
+    [[maybe_unused]] CSN prev_value = csn.exchange(assigned_csn);
+    assert(prev_value == Tx::CommittingCSN);
+    for (const auto & part : creating_parts)
+    {
+        part->version.creation_csn.store(csn);
+        part->appendCSNToVersionMetadata(VersionMetadata::WhichCSN::CREATION);
+    }
+
+    for (const auto & part : removing_parts)
+    {
+        part->version.removal_csn.store(csn);
+        part->appendCSNToVersionMetadata(VersionMetadata::WhichCSN::REMOVAL);
+    }
+
+    for (const auto & storage_and_mutation : mutations)
+        storage_and_mutation.first->setMutationCSN(storage_and_mutation.second, csn);
+}
+
+bool MergeTreeTransaction::rollback() noexcept
+{
+    CSN expected = Tx::UnknownCSN;
+    bool need_rollback = csn.compare_exchange_strong(expected, Tx::RolledBackCSN);
+
+    /// Check that it was not rolled back concurrently
+    if (!need_rollback)
+        return false;
+
+    /// It's not a problem if server crash at this point
+    /// because on startup we will see that TID is not committed and will simply discard these changes.
+
+    RunningMutationsList mutations_to_kill;
+    DataPartsVector parts_to_remove;
+    DataPartsVector parts_to_activate;
+
+    {
+        std::lock_guard lock{mutex};
+        mutations_to_kill = mutations;
+        parts_to_remove = creating_parts;
+        parts_to_activate = removing_parts;
+    }
+
+    /// Forcefully stop related mutations if any
+    for (const auto & table_and_mutation : mutations_to_kill)
+        table_and_mutation.first->killMutation(table_and_mutation.second);
+
+    /// Discard changes in active parts set
+    /// Remove parts that were created, restore parts that were removed (except parts that were created by this transaction too)
+    for (const auto & part : parts_to_remove)
+    {
+        if (part->version.isRemovalTIDLocked())
+        {
+            /// Don't need to remove part from working set if it was created and removed by this transaction
+            assert(part->version.removal_tid_lock == tid.getHash());
+            continue;
+        }
+        /// FIXME do not lock removal_tid when rolling back part creation, it's ugly
+        const_cast<MergeTreeData &>(part->storage).removePartsFromWorkingSet(NO_TRANSACTION_RAW, {part}, true);
+    }
+
+    for (const auto & part : parts_to_activate)
+        if (part->version.getCreationTID() != tid)
+            const_cast<MergeTreeData &>(part->storage).restoreAndActivatePart(part);
+
+    /// Kind of optimization: cleanup thread can remove these parts immediately
+    for (const auto & part : parts_to_remove)
+    {
+        part->version.creation_csn.store(Tx::RolledBackCSN);
+        /// Write special RolledBackCSN, so we will be able to cleanup transaction log
+        part->appendCSNToVersionMetadata(VersionMetadata::CREATION);
+    }
+
+    for (const auto & part : parts_to_activate)
+    {
+        /// Clear removal_tid from version metadata file, so we will not need to distinguish TIDs that were not committed
+        /// and TIDs that were committed long time ago and were removed from the log on log cleanup.
+        part->appendRemovalTIDToVersionMetadata(/* clear */ true);
+        part->version.unlockRemovalTID(tid, TransactionInfoContext{part->storage.getStorageID(), part->name});
+    }
+
+
+    assert([&]()
+    {
+        std::lock_guard lock{mutex};
+        assert(mutations_to_kill == mutations);
+        assert(parts_to_remove == creating_parts);
+        assert(parts_to_activate == removing_parts);
+        return csn == Tx::RolledBackCSN;
+    }());
+
+    return true;
+}
+
+void MergeTreeTransaction::onException()
+{
+    TransactionLog::instance().rollbackTransaction(shared_from_this());
+}
+
+String MergeTreeTransaction::dumpDescription() const
+{
+    String res = fmt::format("{} state: {}, snapshot: {}", tid, getState(), snapshot);
+
+    if (isReadOnly())
+    {
+        res += ", readonly";
+        return res;
+    }
+
+    std::lock_guard lock{mutex};
+
+    res += fmt::format(", affects {} tables:", storages.size());
+
+    using ChangesInTable = std::tuple<Strings, Strings, Strings>;
+    std::unordered_map<const IStorage *, ChangesInTable> storage_to_changes;
+
+    for (const auto & part : creating_parts)
+        std::get<0>(storage_to_changes[&(part->storage)]).push_back(part->name);
+
+    for (const auto & part : removing_parts)
+    {
+        String info = fmt::format("{} (created by {}, {})", part->name, part->version.getCreationTID(), part->version.creation_csn);
+        std::get<1>(storage_to_changes[&(part->storage)]).push_back(std::move(info));
+        assert(!part->version.creation_csn || part->version.creation_csn <= snapshot);
+    }
+
+    for (const auto & mutation : mutations)
+        std::get<2>(storage_to_changes[mutation.first.get()]).push_back(mutation.second);
+
+    for (const auto & storage_changes : storage_to_changes)
+    {
+        res += fmt::format("\n\t{}:", storage_changes.first->getStorageID().getNameForLogs());
+        const auto & creating_info = std::get<0>(storage_changes.second);
+        const auto & removing_info = std::get<1>(storage_changes.second);
+        const auto & mutations_info = std::get<2>(storage_changes.second);
+
+        if (!creating_info.empty())
+            res += fmt::format("\n\t\tcreating parts:\n\t\t\t{}", fmt::join(creating_info, "\n\t\t\t"));
+        if (!removing_info.empty())
+            res += fmt::format("\n\t\tremoving parts:\n\t\t\t{}", fmt::join(removing_info, "\n\t\t\t"));
+        if (!mutations_info.empty())
+            res += fmt::format("\n\t\tmutations:\n\t\t\t{}", fmt::join(mutations_info, "\n\t\t\t"));
+    }
+
+    return res;
+}
+
+}

src/Interpreters/MergeTreeTransaction.h

@@ -0,0 +1,84 @@
+#pragma once
+#include <Interpreters/TransactionVersionMetadata.h>
+#include <boost/noncopyable.hpp>
+#include <Storages/IStorage_fwd.h>
+#include <Storages/TableLockHolder.h>
+#include <Common/Stopwatch.h>
+#include <base/scope_guard.h>
+
+#include <list>
+#include <unordered_set>
+
+namespace DB
+{
+
+class IMergeTreeDataPart;
+using DataPartPtr = std::shared_ptr<const IMergeTreeDataPart>;
+using DataPartsVector = std::vector<DataPartPtr>;
+
+/// This object is responsible for tracking all changes that some transaction is making in MergeTree tables.
+/// It collects all changes that queries of current transaction made in data part sets of all MergeTree tables
+/// to ether make them visible when transaction commits or undo when transaction rolls back.
+class MergeTreeTransaction : public std::enable_shared_from_this<MergeTreeTransaction>, private boost::noncopyable
+{
+    friend class TransactionLog;
+public:
+    enum State
+    {
+        RUNNING,
+        COMMITTED,
+        ROLLED_BACK,
+    };
+
+    CSN getSnapshot() const { return snapshot; }
+    void setSnapshot(CSN new_snapshot);
+    State getState() const;
+
+    const TransactionID tid;
+
+    MergeTreeTransaction(CSN snapshot_, LocalTID local_tid_, UUID host_id);
+
+    void addNewPart(const StoragePtr & storage, const DataPartPtr & new_part);
+    void removeOldPart(const StoragePtr & storage, const DataPartPtr & part_to_remove, const TransactionInfoContext & context);
+
+    void addMutation(const StoragePtr & table, const String & mutation_id);
+
+    static void addNewPart(const StoragePtr & storage, const DataPartPtr & new_part, MergeTreeTransaction * txn);
+    static void removeOldPart(const StoragePtr & storage, const DataPartPtr & part_to_remove, MergeTreeTransaction * txn);
+    static void addNewPartAndRemoveCovered(const StoragePtr & storage, const DataPartPtr & new_part, const DataPartsVector & covered_parts, MergeTreeTransaction * txn);
+
+    bool isReadOnly() const;
+
+    void onException();
+
+    String dumpDescription() const;
+
+    Float64 elapsedSeconds() const { return elapsed.elapsedSeconds(); }
+
+private:
+    scope_guard beforeCommit();
+    void afterCommit(CSN assigned_csn) noexcept;
+    bool rollback() noexcept;
+    void checkIsNotCancelled() const;
+
+    mutable std::mutex mutex;
+    Stopwatch elapsed;
+
+    /// Usually it's equal to tid.start_csn, but can be changed by SET SNAPSHOT query (for introspection purposes and time-traveling)
+    CSN snapshot;
+    std::list<CSN>::iterator snapshot_in_use_it;
+
+    /// Lists of changes made by transaction
+    std::unordered_set<StoragePtr> storages;
+    std::vector<TableLockHolder> table_read_locks_for_ordinary_db;
+    DataPartsVector creating_parts;
+    DataPartsVector removing_parts;
+    using RunningMutationsList = std::vector<std::pair<StoragePtr, String>>;
+    RunningMutationsList mutations;
+
+    std::atomic<CSN> csn;
+};
+
+using MergeTreeTransactionPtr = std::shared_ptr<MergeTreeTransaction>;
+
+}

src/Interpreters/MergeTreeTransactionHolder.cpp

@@ -0,0 +1,84 @@
+#include <Interpreters/MergeTreeTransactionHolder.h>
+#include <Interpreters/MergeTreeTransaction.h>
+#include <Interpreters/TransactionLog.h>
+#include <Interpreters/Context.h>
+
+namespace DB
+{
+
+namespace ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+}
+
+MergeTreeTransactionHolder::MergeTreeTransactionHolder(const MergeTreeTransactionPtr & txn_, bool autocommit_ = false, const Context * owned_by_session_context_)
+    : txn(txn_)
+    , autocommit(autocommit_)
+    , owned_by_session_context(owned_by_session_context_)
+{
+    assert(!txn || txn->getState() == MergeTreeTransaction::RUNNING);
+    assert(!owned_by_session_context || owned_by_session_context == owned_by_session_context->getSessionContext().get());
+}
+
+MergeTreeTransactionHolder::MergeTreeTransactionHolder(MergeTreeTransactionHolder && rhs) noexcept
+{
+    *this = std::move(rhs);
+}
+
+MergeTreeTransactionHolder & MergeTreeTransactionHolder::operator=(MergeTreeTransactionHolder && rhs) noexcept
+{
+    onDestroy();
+    txn = NO_TRANSACTION_PTR;
+    autocommit = false;
+    owned_by_session_context = nullptr;
+    std::swap(txn, rhs.txn);
+    std::swap(autocommit, rhs.autocommit);
+    std::swap(owned_by_session_context, rhs.owned_by_session_context);
+    return *this;
+}
+
+MergeTreeTransactionHolder::~MergeTreeTransactionHolder()
+{
+    onDestroy();
+}
+
+void MergeTreeTransactionHolder::onDestroy() noexcept
+{
+    if (!txn)
+        return;
+    if (txn->getState() != MergeTreeTransaction::RUNNING)
+        return;
+
+    if (autocommit && std::uncaught_exceptions() == 0)
+    {
+        try
+        {
+            TransactionLog::instance().commitTransaction(txn);
+            return;
+        }
+        catch (...)
+        {
+            tryLogCurrentException(__PRETTY_FUNCTION__);
+        }
+    }
+
+    TransactionLog::instance().rollbackTransaction(txn);
+}
+
+MergeTreeTransactionHolder::MergeTreeTransactionHolder(const MergeTreeTransactionHolder & rhs)
+{
+    *this = rhs;
+}
+
+MergeTreeTransactionHolder & MergeTreeTransactionHolder::operator=(const MergeTreeTransactionHolder & rhs)  // NOLINT
+{
+    if (rhs.txn && !rhs.owned_by_session_context)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+                        "Tried to copy non-empty MergeTreeTransactionHolder that is not owned by session context. It's a bug");
+    assert(!txn);
+    assert(!autocommit);
+    assert(!owned_by_session_context);
+    return *this;
+}
+
+}

src/Interpreters/MergeTreeTransactionHolder.h

@@ -0,0 +1,42 @@
+#pragma once
+#include <memory>
+
+namespace DB
+{
+
+class Context;
+
+class MergeTreeTransaction;
+/// TODO maybe replace with raw pointer? It should not be shared, only MergeTreeTransactionHolder can own a transaction object
+using MergeTreeTransactionPtr = std::shared_ptr<MergeTreeTransaction>;
+
+/// Owns a MergeTreeTransactionObject.
+/// Rolls back a transaction in dtor if it was not committed.
+/// If `autocommit` flag is true, then it commits transaction if dtor is called normally
+/// or rolls it back if dtor was called due to an exception.
+class MergeTreeTransactionHolder
+{
+public:
+    MergeTreeTransactionHolder() = default;
+    MergeTreeTransactionHolder(const MergeTreeTransactionPtr & txn_, bool autocommit_, const Context * owned_by_session_context_ = nullptr);
+    MergeTreeTransactionHolder(MergeTreeTransactionHolder && rhs) noexcept;
+    MergeTreeTransactionHolder & operator=(MergeTreeTransactionHolder && rhs) noexcept;
+    ~MergeTreeTransactionHolder();
+
+    /// NOTE: We cannot make it noncopyable, because we use it as a field of Context.
+    /// So the following copy constructor and operator does not copy anything,
+    /// they just leave txn nullptr.
+    MergeTreeTransactionHolder(const MergeTreeTransactionHolder & rhs);
+    MergeTreeTransactionHolder & operator=(const MergeTreeTransactionHolder & rhs);
+
+    MergeTreeTransactionPtr getTransaction() const { return txn; }
+
+private:
+    void onDestroy() noexcept;
+
+    MergeTreeTransactionPtr txn;
+    bool autocommit = false;
+    const Context * owned_by_session_context = nullptr;
+};
+
+}

src/Interpreters/MutationsInterpreter.cpp

@@ -802,7 +802,7 @@ ASTPtr MutationsInterpreter::prepareInterpreterSelectQuery(std::vector<Stage> &
         /// e.g. ALTER referencing the same table in scalar subquery
         bool execute_scalar_subqueries = !dry_run;
         auto syntax_result = TreeRewriter(context).analyze(
-            all_asts, all_columns, storage, storage->getStorageSnapshot(metadata_snapshot),
+            all_asts, all_columns, storage, storage->getStorageSnapshot(metadata_snapshot, context),
             false, true, execute_scalar_subqueries);
 
         if (execute_scalar_subqueries && context->hasQueryContext())

src/Interpreters/QueryLog.cpp

@@ -117,7 +117,9 @@ NamesAndTypesList QueryLogElement::getNamesAndTypes()
         {"used_formats", std::make_shared<DataTypeArray>(std::make_shared<DataTypeString>())},
         {"used_functions", std::make_shared<DataTypeArray>(std::make_shared<DataTypeString>())},
         {"used_storages", std::make_shared<DataTypeArray>(std::make_shared<DataTypeString>())},
-        {"used_table_functions", std::make_shared<DataTypeArray>(std::make_shared<DataTypeString>())}
+        {"used_table_functions", std::make_shared<DataTypeArray>(std::make_shared<DataTypeString>())},
+
+        {"transaction_id", getTransactionIDDataType()},
     };
 
 }
@@ -257,6 +259,8 @@ void QueryLogElement::appendToBlock(MutableColumns & columns) const
         fill_column(used_storages, column_storage_factory_objects);
         fill_column(used_table_functions, column_table_function_factory_objects);
     }
+
+    columns[i++]->insert(Tuple{tid.start_csn, tid.local_tid, tid.host_id});
 }
 
 void QueryLogElement::appendClientInfo(const ClientInfo & client_info, MutableColumns & columns, size_t & i)

src/Interpreters/QueryLog.h

@@ -5,6 +5,7 @@
 #include <Core/Settings.h>
 #include <Interpreters/SystemLog.h>
 #include <Interpreters/ClientInfo.h>
+#include <Interpreters/TransactionVersionMetadata.h>
 
 namespace ProfileEvents
 {
@@ -85,6 +86,8 @@ struct QueryLogElement
     std::shared_ptr<ProfileEvents::Counters::Snapshot> profile_counters;
     std::shared_ptr<Settings> query_settings;
 
+    TransactionID tid;
+
     static std::string name() { return "QueryLog"; }
 
     static NamesAndTypesList getNamesAndTypes();

src/Interpreters/SystemLog.cpp

@@ -11,6 +11,7 @@
 #include <Interpreters/TraceLog.h>
 #include <Interpreters/ProcessorsProfileLog.h>
 #include <Interpreters/ZooKeeperLog.h>
+#include <Interpreters/TransactionsInfoLog.h>
 #include <Interpreters/InterpreterCreateQuery.h>
 #include <Interpreters/InterpreterRenameQuery.h>
 #include <Interpreters/InterpreterInsertQuery.h>
@@ -202,6 +203,8 @@ SystemLogs::SystemLogs(ContextPtr global_context, const Poco::Util::AbstractConf
     query_views_log = createSystemLog<QueryViewsLog>(global_context, "system", "query_views_log", config, "query_views_log");
     zookeeper_log = createSystemLog<ZooKeeperLog>(global_context, "system", "zookeeper_log", config, "zookeeper_log");
     session_log = createSystemLog<SessionLog>(global_context, "system", "session_log", config, "session_log");
+    transactions_info_log = createSystemLog<TransactionsInfoLog>(
+        global_context, "system", "transactions_info_log", config, "transactions_info_log");
     processors_profile_log = createSystemLog<ProcessorsProfileLog>(global_context, "system", "processors_profile_log", config, "processors_profile_log");
 
     if (query_log)
@@ -228,6 +231,8 @@ SystemLogs::SystemLogs(ContextPtr global_context, const Poco::Util::AbstractConf
         logs.emplace_back(zookeeper_log.get());
     if (session_log)
         logs.emplace_back(session_log.get());
+    if (transactions_info_log)
+        logs.emplace_back(transactions_info_log.get());
     if (processors_profile_log)
         logs.emplace_back(processors_profile_log.get());
 
@@ -544,6 +549,7 @@ ASTPtr SystemLog<LogElement>::getCreateTableQuery()
     return create;
 }
 
+
 #define INSTANTIATE_SYSTEM_LOG(ELEMENT) template class SystemLog<ELEMENT>;
 SYSTEM_LOG_ELEMENTS(INSTANTIATE_SYSTEM_LOG)
 

src/Interpreters/SystemLog.h

@@ -43,6 +43,7 @@ class OpenTelemetrySpanLog;
 class QueryViewsLog;
 class ZooKeeperLog;
 class SessionLog;
+class TransactionsInfoLog;
 class ProcessorsProfileLog;
 
 /// System logs should be destroyed in destructor of the last Context and before tables,
@@ -71,6 +72,8 @@ struct SystemLogs
     std::shared_ptr<ZooKeeperLog> zookeeper_log;
     /// Login, LogOut and Login failure events
     std::shared_ptr<SessionLog> session_log;
+    /// Events related to transactions
+    std::shared_ptr<TransactionsInfoLog> transactions_info_log;
     /// Used to log processors profiling
     std::shared_ptr<ProcessorsProfileLog> processors_profile_log;
 

src/Interpreters/TransactionLog.cpp

@@ -0,0 +1,484 @@
+#include <Interpreters/TransactionLog.h>
+#include <Interpreters/TransactionVersionMetadata.h>
+#include <Interpreters/Context.h>
+#include <Interpreters/TransactionsInfoLog.h>
+#include <IO/ReadHelpers.h>
+#include <IO/WriteHelpers.h>
+#include <IO/ReadBufferFromString.h>
+#include <Common/Exception.h>
+#include <Common/ZooKeeper/KeeperException.h>
+#include <Core/ServerUUID.h>
+#include <base/logger_useful.h>
+
+
+/// It's used in critical places to exit on unexpected exceptions.
+/// SIGABRT is usually better that broken state in memory with unpredictable consequences.
+#define NOEXCEPT_SCOPE SCOPE_EXIT({ if (std::uncaught_exceptions()) { tryLogCurrentException("NOEXCEPT_SCOPE"); abort(); } })
+
+namespace DB
+{
+
+namespace ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+}
+
+static void tryWriteEventToSystemLog(Poco::Logger * log, ContextPtr context,
+                                     TransactionsInfoLogElement::Type type, const TransactionID & tid, CSN csn = Tx::UnknownCSN)
+try
+{
+    auto system_log = context->getTransactionsInfoLog();
+    if (!system_log)
+        return;
+
+    TransactionsInfoLogElement elem;
+    elem.type = type;
+    elem.tid = tid;
+    elem.csn = csn;
+    elem.fillCommonFields(nullptr);
+    system_log->add(elem);
+}
+catch (...)
+{
+    tryLogCurrentException(log);
+}
+
+
+TransactionLog::TransactionLog()
+    : log(&Poco::Logger::get("TransactionLog"))
+{
+    global_context = Context::getGlobalContextInstance();
+    global_context->checkTransactionsAreAllowed();
+
+    zookeeper_path = global_context->getConfigRef().getString("transaction_log.zookeeper_path", "/clickhouse/txn");
+    zookeeper_path_log = zookeeper_path + "/log";
+
+    loadLogFromZooKeeper();
+
+    updating_thread = ThreadFromGlobalPool(&TransactionLog::runUpdatingThread, this);
+}
+
+TransactionLog::~TransactionLog()
+{
+    shutdown();
+}
+
+void TransactionLog::shutdown()
+{
+    if (stop_flag.exchange(true))
+        return;
+    log_updated_event->set();
+    latest_snapshot.notify_all();
+    updating_thread.join();
+
+    std::lock_guard lock{mutex};
+    /// This is required to... you'll never guess - avoid race condition inside Poco::Logger (Coordination::ZooKeeper::log)
+    zookeeper.reset();
+}
+
+ZooKeeperPtr TransactionLog::getZooKeeper() const
+{
+    std::lock_guard lock{mutex};
+    return zookeeper;
+}
+
+UInt64 TransactionLog::deserializeCSN(const String & csn_node_name)
+{
+    ReadBufferFromString buf{csn_node_name};
+    assertString("csn-", buf);
+    UInt64 res;
+    readText(res, buf);
+    assertEOF(buf);
+    return res;
+}
+
+String TransactionLog::serializeCSN(CSN csn)
+{
+    return zkutil::getSequentialNodeName("csn-", csn);
+}
+
+TransactionID TransactionLog::deserializeTID(const String & csn_node_content)
+{
+    TransactionID tid = Tx::EmptyTID;
+    if (csn_node_content.empty())
+        return tid;
+
+    ReadBufferFromString buf{csn_node_content};
+    tid = TransactionID::read(buf);
+    assertEOF(buf);
+    return tid;
+}
+
+String TransactionLog::serializeTID(const TransactionID & tid)
+{
+    WriteBufferFromOwnString buf;
+    TransactionID::write(tid, buf);
+    return buf.str();
+}
+
+
+void TransactionLog::loadEntries(Strings::const_iterator beg, Strings::const_iterator end)
+{
+    std::vector<std::future<Coordination::GetResponse>> futures;
+    size_t entries_count = std::distance(beg, end);
+    if (!entries_count)
+        return;
+
+    String last_entry = *std::prev(end);
+    LOG_TRACE(log, "Loading {} entries from {}: {}..{}", entries_count, zookeeper_path_log, *beg, last_entry);
+    futures.reserve(entries_count);
+    for (auto it = beg; it != end; ++it)
+        futures.emplace_back(zookeeper->asyncGet(fs::path(zookeeper_path_log) / *it));
+
+    std::vector<std::pair<TIDHash, CSNEntry>> loaded;
+    loaded.reserve(entries_count);
+    auto it = beg;
+    for (size_t i = 0; i < entries_count; ++i, ++it)
+    {
+        auto res = futures[i].get();
+        CSN csn = deserializeCSN(*it);
+        TransactionID tid = deserializeTID(res.data);
+        loaded.emplace_back(tid.getHash(), CSNEntry{csn, tid});
+        LOG_TEST(log, "Got entry {} -> {}", tid, csn);
+    }
+    futures.clear();
+
+    NOEXCEPT_SCOPE;
+    LockMemoryExceptionInThread lock_memory_tracker(VariableContext::Global);
+    std::lock_guard lock{mutex};
+    for (const auto & entry : loaded)
+    {
+        if (entry.first == Tx::EmptyTID.getHash())
+            continue;
+
+        tid_to_csn.emplace(entry.first, entry.second);
+    }
+    last_loaded_entry = last_entry;
+    latest_snapshot = loaded.back().second.csn;
+    local_tid_counter = Tx::MaxReservedLocalTID;
+}
+
+void TransactionLog::loadLogFromZooKeeper()
+{
+    assert(!zookeeper);
+    assert(tid_to_csn.empty());
+    assert(last_loaded_entry.empty());
+    zookeeper = global_context->getZooKeeper();
+
+    /// We do not write local_tid_counter to disk or zk and maintain it only in memory.
+    /// Create empty entry to allocate new CSN to safely start counting from the beginning and avoid TID duplication.
+    /// TODO It's possible to skip this step in come cases (especially for multi-host configuration).
+    Coordination::Error code = zookeeper->tryCreate(zookeeper_path_log + "/csn-", "", zkutil::CreateMode::PersistentSequential);
+    if (code != Coordination::Error::ZOK)
+    {
+        /// Log probably does not exist, create it
+        assert(code == Coordination::Error::ZNONODE);
+        zookeeper->createAncestors(zookeeper_path_log);
+        Coordination::Requests ops;
+        ops.emplace_back(zkutil::makeCreateRequest(zookeeper_path + "/tail_ptr", serializeCSN(Tx::MaxReservedCSN), zkutil::CreateMode::Persistent));
+        ops.emplace_back(zkutil::makeCreateRequest(zookeeper_path_log, "", zkutil::CreateMode::Persistent));
+
+        /// Fast-forward sequential counter to skip reserved CSNs
+        for (size_t i = 0; i <= Tx::MaxReservedCSN; ++i)
+            ops.emplace_back(zkutil::makeCreateRequest(zookeeper_path_log + "/csn-", "", zkutil::CreateMode::PersistentSequential));
+        Coordination::Responses res;
+        code = zookeeper->tryMulti(ops, res);
+        if (code != Coordination::Error::ZNODEEXISTS)
+            zkutil::KeeperMultiException::check(code, ops, res);
+    }
+
+    /// TODO Split log into "subdirectories" to:
+    /// 1. fetch it more optimal way (avoid listing all CSNs on further incremental updates)
+    /// 2. simplify log rotation
+    /// 3. support 64-bit CSNs on top of Apache ZooKeeper (it uses Int32 for sequential numbers)
+    Strings entries_list = zookeeper->getChildren(zookeeper_path_log, nullptr, log_updated_event);
+    assert(!entries_list.empty());
+    std::sort(entries_list.begin(), entries_list.end());
+    loadEntries(entries_list.begin(), entries_list.end());
+    assert(!last_loaded_entry.empty());
+    assert(latest_snapshot == deserializeCSN(last_loaded_entry));
+    local_tid_counter = Tx::MaxReservedLocalTID;
+
+    tail_ptr = deserializeCSN(zookeeper->get(zookeeper_path + "/tail_ptr"));
+}
+
+void TransactionLog::runUpdatingThread()
+{
+    while (true)
+    {
+        try
+        {
+            log_updated_event->wait();
+            if (stop_flag.load())
+                return;
+
+            if (!zookeeper)
+            {
+                auto new_zookeeper = global_context->getZooKeeper();
+                std::lock_guard lock{mutex};
+                zookeeper = new_zookeeper;
+            }
+
+            loadNewEntries();
+            removeOldEntries();
+        }
+        catch (const Coordination::Exception & e)
+        {
+            tryLogCurrentException(log);
+            /// TODO better backoff
+            std::this_thread::sleep_for(std::chrono::milliseconds(1000));
+            if (Coordination::isHardwareError(e.code))
+            {
+                std::lock_guard lock{mutex};
+                zookeeper.reset();
+            }
+            log_updated_event->set();
+        }
+        catch (...)
+        {
+            tryLogCurrentException(log);
+            std::this_thread::sleep_for(std::chrono::milliseconds(1000));
+            log_updated_event->set();
+        }
+    }
+}
+
+void TransactionLog::loadNewEntries()
+{
+    Strings entries_list = zookeeper->getChildren(zookeeper_path_log, nullptr, log_updated_event);
+    assert(!entries_list.empty());
+    std::sort(entries_list.begin(), entries_list.end());
+    auto it = std::upper_bound(entries_list.begin(), entries_list.end(), last_loaded_entry);
+    loadEntries(it, entries_list.end());
+    assert(last_loaded_entry == entries_list.back());
+    assert(latest_snapshot == deserializeCSN(last_loaded_entry));
+    latest_snapshot.notify_all();
+}
+
+void TransactionLog::removeOldEntries()
+{
+    /// Try to update tail pointer. It's (almost) safe to set it to the oldest snapshot
+    /// because if a transaction released snapshot, then CSN is already written into metadata.
+    /// Why almost? Because on server startup we do not have the oldest snapshot (it's simply equal to the latest one),
+    /// but it's possible that some CSNs are not written into data parts (and we will write them during startup).
+    if (!global_context->isServerCompletelyStarted())
+        return;
+
+    /// Also similar problem is possible if some table was not attached during startup (for example, if table is detached permanently).
+    /// Also we write CSNs into data parts without fsync, so it's theoretically possible that we wrote CSN, finished transaction,
+    /// removed its entry from the log, but after that server restarts and CSN is not actually saved to metadata on disk.
+    /// We should store a bit more entries in ZK and keep outdated entries for a while.
+
+    /// TODO we will need a bit more complex logic for multiple hosts
+    Coordination::Stat stat;
+    CSN old_tail_ptr = deserializeCSN(zookeeper->get(zookeeper_path + "/tail_ptr", &stat));
+    CSN new_tail_ptr = getOldestSnapshot();
+    if (new_tail_ptr < old_tail_ptr)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "Got unexpected tail_ptr {}, oldest snapshot is {}, it's a bug", old_tail_ptr, new_tail_ptr);
+    else if (new_tail_ptr == old_tail_ptr)
+        return;
+
+    /// (it's not supposed to fail with ZBADVERSION while there is only one host)
+    LOG_TRACE(log, "Updating tail_ptr from {} to {}", old_tail_ptr, new_tail_ptr);
+    zookeeper->set(zookeeper_path + "/tail_ptr", serializeCSN(new_tail_ptr), stat.version);
+    tail_ptr.store(new_tail_ptr);
+
+    /// Now we can find and remove old entries
+    TIDMap tids;
+    {
+        std::lock_guard lock{mutex};
+        tids = tid_to_csn;
+    }
+
+    /// TODO support batching
+    std::vector<TIDHash> removed_entries;
+    CSN latest_entry_csn = latest_snapshot.load();
+    for (const auto & elem : tids)
+    {
+        /// Definitely not safe to remove
+        if (new_tail_ptr <= elem.second.tid.start_csn)
+            continue;
+
+        /// Keep at least one node (the latest one we fetched)
+        if (elem.second.csn == latest_entry_csn)
+            continue;
+
+        LOG_TEST(log, "Removing entry {} -> {}", elem.second.tid, elem.second.csn);
+        auto code = zookeeper->tryRemove(zookeeper_path_log + "/" + serializeCSN(elem.second.csn));
+        if (code == Coordination::Error::ZOK || code == Coordination::Error::ZNONODE)
+            removed_entries.push_back(elem.first);
+    }
+
+    std::lock_guard lock{mutex};
+    for (const auto & tid_hash : removed_entries)
+        tid_to_csn.erase(tid_hash);
+}
+
+CSN TransactionLog::getLatestSnapshot() const
+{
+    return latest_snapshot.load();
+}
+
+MergeTreeTransactionPtr TransactionLog::beginTransaction()
+{
+    MergeTreeTransactionPtr txn;
+    {
+        std::lock_guard lock{running_list_mutex};
+        CSN snapshot = latest_snapshot.load();
+        LocalTID ltid = 1 + local_tid_counter.fetch_add(1);
+        txn = std::make_shared<MergeTreeTransaction>(snapshot, ltid, ServerUUID::get());
+        bool inserted = running_list.try_emplace(txn->tid.getHash(), txn).second;
+        if (!inserted)
+            throw Exception(ErrorCodes::LOGICAL_ERROR, "I's a bug: TID {} {} exists", txn->tid.getHash(), txn->tid);
+        txn->snapshot_in_use_it = snapshots_in_use.insert(snapshots_in_use.end(), snapshot);
+    }
+
+    LOG_TEST(log, "Beginning transaction {} ({})", txn->tid, txn->tid.getHash());
+    tryWriteEventToSystemLog(log, global_context, TransactionsInfoLogElement::BEGIN, txn->tid);
+
+    return txn;
+}
+
+CSN TransactionLog::commitTransaction(const MergeTreeTransactionPtr & txn)
+{
+    /// Some precommit checks, may throw
+    auto committing_lock = txn->beforeCommit();
+
+    CSN new_csn;
+    if (txn->isReadOnly())
+    {
+        /// Don't need to allocate CSN in ZK for readonly transactions, it's safe to use snapshot/start_csn as "commit" timestamp
+        LOG_TEST(log, "Closing readonly transaction {}", txn->tid);
+        new_csn = txn->snapshot;
+        tryWriteEventToSystemLog(log, global_context, TransactionsInfoLogElement::COMMIT, txn->tid, new_csn);
+    }
+    else
+    {
+        LOG_TEST(log, "Committing transaction {}", txn->dumpDescription());
+        /// TODO handle connection loss
+        /// TODO support batching
+        auto current_zookeeper = getZooKeeper();
+        String path_created = current_zookeeper->create(zookeeper_path_log + "/csn-", serializeTID(txn->tid), zkutil::CreateMode::PersistentSequential);    /// Commit point
+        NOEXCEPT_SCOPE;
+
+        /// FIXME Transactions: Sequential node numbers in ZooKeeper are Int32, but 31 bit is not enough for production use
+        /// (overflow is possible in a several weeks/months of active usage)
+        new_csn = deserializeCSN(path_created.substr(zookeeper_path_log.size() + 1));
+
+        LOG_INFO(log, "Transaction {} committed with CSN={}", txn->tid, new_csn);
+        tryWriteEventToSystemLog(log, global_context, TransactionsInfoLogElement::COMMIT, txn->tid, new_csn);
+
+        /// Wait for committed changes to become actually visible, so the next transaction in this session will see the changes
+        /// TODO it's optional, add a setting for this
+        auto current_latest_snapshot = latest_snapshot.load();
+        while (current_latest_snapshot < new_csn && !stop_flag)
+        {
+            latest_snapshot.wait(current_latest_snapshot);
+            current_latest_snapshot = latest_snapshot.load();
+        }
+    }
+
+    /// Write allocated CSN, so we will be able to cleanup log in ZK. This method is noexcept.
+    txn->afterCommit(new_csn);
+
+    {
+        /// Finally we can remove transaction from the list and release the snapshot
+        std::lock_guard lock{running_list_mutex};
+        bool removed = running_list.erase(txn->tid.getHash());
+        if (!removed)
+            throw Exception(ErrorCodes::LOGICAL_ERROR, "I's a bug: TID {} {} doesn't exist", txn->tid.getHash(), txn->tid);
+        snapshots_in_use.erase(txn->snapshot_in_use_it);
+    }
+
+    return new_csn;
+}
+
+void TransactionLog::rollbackTransaction(const MergeTreeTransactionPtr & txn) noexcept
+{
+    LOG_TRACE(log, "Rolling back transaction {}{}", txn->tid,
+              std::uncaught_exceptions() ? fmt::format(" due to uncaught exception (code: {})", getCurrentExceptionCode()) : "");
+
+    if (!txn->rollback())
+    {
+        /// Transaction was cancelled concurrently, it's already rolled back.
+        assert(txn->csn == Tx::RolledBackCSN);
+        return;
+    }
+
+    {
+        std::lock_guard lock{running_list_mutex};
+        bool removed = running_list.erase(txn->tid.getHash());
+        if (!removed)
+            abort();
+        snapshots_in_use.erase(txn->snapshot_in_use_it);
+    }
+
+    tryWriteEventToSystemLog(log, global_context, TransactionsInfoLogElement::ROLLBACK, txn->tid);
+}
+
+MergeTreeTransactionPtr TransactionLog::tryGetRunningTransaction(const TIDHash & tid)
+{
+    std::lock_guard lock{running_list_mutex};
+    auto it = running_list.find(tid);
+    if (it == running_list.end())
+        return NO_TRANSACTION_PTR;
+    return it->second;
+}
+
+CSN TransactionLog::getCSN(const TransactionID & tid)
+{
+    /// Avoid creation of the instance if transactions are not actually involved
+    if (tid == Tx::PrehistoricTID)
+        return Tx::PrehistoricCSN;
+    return instance().getCSNImpl(tid.getHash());
+}
+
+CSN TransactionLog::getCSN(const TIDHash & tid)
+{
+    /// Avoid creation of the instance if transactions are not actually involved
+    if (tid == Tx::PrehistoricTID.getHash())
+        return Tx::PrehistoricCSN;
+    return instance().getCSNImpl(tid);
+}
+
+CSN TransactionLog::getCSNImpl(const TIDHash & tid_hash) const
+{
+    assert(tid_hash);
+    assert(tid_hash != Tx::EmptyTID.getHash());
+
+    std::lock_guard lock{mutex};
+    auto it = tid_to_csn.find(tid_hash);
+    if (it != tid_to_csn.end())
+        return it->second.csn;
+
+    return Tx::UnknownCSN;
+}
+
+void TransactionLog::assertTIDIsNotOutdated(const TransactionID & tid)
+{
+    if (tid == Tx::PrehistoricTID)
+        return;
+
+    /// Ensure that we are not trying to get CSN for TID that was already removed from the log
+    CSN tail = instance().tail_ptr.load();
+    if (tail <= tid.start_csn)
+        return;
+
+    throw Exception(ErrorCodes::LOGICAL_ERROR, "Trying to get CSN for too old TID {}, current tail_ptr is {}, probably it's a bug", tid, tail);
+}
+
+CSN TransactionLog::getOldestSnapshot() const
+{
+    std::lock_guard lock{running_list_mutex};
+    if (snapshots_in_use.empty())
+        return getLatestSnapshot();
+    return snapshots_in_use.front();
+}
+
+TransactionLog::TransactionsList TransactionLog::getTransactionsList() const
+{
+    std::lock_guard lock{running_list_mutex};
+    return running_list;
+}
+
+}

src/Interpreters/TransactionLog.h

@@ -0,0 +1,192 @@
+#pragma once
+#include <Interpreters/MergeTreeTransaction.h>
+#include <Interpreters/MergeTreeTransactionHolder.h>
+#include <Common/ZooKeeper/ZooKeeper.h>
+#include <Common/ThreadPool.h>
+#include <boost/noncopyable.hpp>
+#include <mutex>
+#include <unordered_map>
+
+namespace DB
+{
+
+/// We want to create a TransactionLog object lazily and avoid creation if it's not needed.
+/// But we also want to call shutdown() in a specific place to avoid race conditions.
+/// We cannot simply use return-static-variable pattern,
+/// because a call to shutdown() may construct unnecessary object in this case.
+template <typename Derived>
+class SingletonHelper : private boost::noncopyable
+{
+public:
+    static Derived & instance()
+    {
+        Derived * ptr = instance_raw_ptr.load();
+        if (likely(ptr))
+            return *ptr;
+
+        return createInstanceOrThrow();
+    }
+
+    static void shutdownIfAny()
+    {
+        std::lock_guard lock{instance_mutex};
+        if (instance_holder)
+            instance_holder->shutdown();
+    }
+
+private:
+    static Derived & createInstanceOrThrow();
+
+    static inline std::atomic<Derived *> instance_raw_ptr;
+    /// It was supposed to be std::optional, but gcc fails to compile it for some reason
+    static inline std::shared_ptr<Derived> instance_holder;
+    static inline std::mutex instance_mutex;
+};
+
+class TransactionsInfoLog;
+using TransactionsInfoLogPtr = std::shared_ptr<TransactionsInfoLog>;
+using ZooKeeperPtr = std::shared_ptr<zkutil::ZooKeeper>;
+
+/// This class maintains transaction log in ZooKeeper and a list of currently running transactions in memory.
+///
+/// Each transaction has unique ID (TID, see details below).
+/// TransactionID is allocated when transaction begins.
+///
+/// We use TransactionID to associate changes (created/removed data parts) with transaction that has made/is going to make these changes.
+/// To commit a transaction we create sequential node "/path_to_log/log/csn-" in ZK and write TID into this node.
+/// Allocated sequential number is a commit timestamp or Commit Sequence Number (CSN). It indicates a (logical) point in time
+/// when transaction is committed and all its changes became visible. So we have total order of all changes.
+///
+/// Also CSNs are used as snapshots: all changes that were made by a transaction that was committed with a CSN less or equal than some_csn
+/// are visible in some_csn snapshot.
+///
+/// TransactionID consists of three parts: (start_csn, local_tid, host_id)
+///   - start_csn is the newest CSN that existed when the transaction was started and also it's snapshot that is visible for this transaction
+///   - local_tid is local sequential number of the transaction, each server allocates local_tids independently without requests to ZK
+///   - host_id is persistent UUID of host that has started the transaction, it's kind of tie-breaker that makes ID unique across all servers
+///
+/// To check if some transaction is committed or not we fetch "csn-xxxxxx" nodes from ZK and construct TID -> CSN mapping,
+/// so for committed transactions we know commit timestamps.
+/// However, if we did not find a mapping for some TID, it means one of the following cases:
+///    1. Transaction is not committed (yet)
+///    2. Transaction is rolled back (quite similar to the first case, but it will never be committed)
+///    3. Transactions was committed a long time ago and we removed its entry from the log
+/// To distinguish the third case we store a "tail pointer" in "/path_to_log/tail_ptr". It's a CSN such that it's safe to remove from log
+/// entries with tid.start_csn < tail_ptr, because CSNs for those TIDs are already written into data parts
+/// and we will not do a CSN lookup for those TIDs anymore.
+///
+/// (however, transactions involving multiple hosts and/or ReplicatedMergeTree tables are currently not supported)
+class TransactionLog final : public SingletonHelper<TransactionLog>
+{
+public:
+
+    TransactionLog();
+
+    ~TransactionLog();
+
+    void shutdown();
+
+    /// Returns the newest snapshot available for reading
+    CSN getLatestSnapshot() const;
+    /// Returns the oldest snapshot that is visible for some running transaction
+    CSN getOldestSnapshot() const;
+
+    /// Allocates TID, returns new transaction object
+    MergeTreeTransactionPtr beginTransaction();
+
+    /// Tries to commit transaction. Returns Commit Sequence Number.
+    /// Throw if transaction was concurrently killed or if some precommit check failed.
+    /// May throw if ZK connection is lost. Transaction status is unknown in this case.
+    CSN commitTransaction(const MergeTreeTransactionPtr & txn);
+
+    /// Releases locks that that were acquired by transaction, releases snapshot, removes transaction from the list of active transactions.
+    /// Normally it should not throw, but if it does for some reason (global memory limit exceeded, disk failure, etc)
+    /// then we should terminate server and reinitialize it to avoid corruption of data structures. That's why it's noexcept.
+    void rollbackTransaction(const MergeTreeTransactionPtr & txn) noexcept;
+
+    /// Returns CSN if transaction with specified ID was committed and UnknownCSN if it was not.
+    /// Returns PrehistoricCSN for PrehistoricTID without creating a TransactionLog instance as a special case.
+    static CSN getCSN(const TransactionID & tid);
+    static CSN getCSN(const TIDHash & tid);
+
+    /// Ensures that getCSN returned UnknownCSN because transaction is not committed and not because entry was removed from the log.
+    static void assertTIDIsNotOutdated(const TransactionID & tid);
+
+    /// Returns a pointer to transaction object if it's running or nullptr.
+    MergeTreeTransactionPtr tryGetRunningTransaction(const TIDHash & tid);
+
+    using TransactionsList = std::unordered_map<TIDHash, MergeTreeTransactionPtr>;
+    /// Returns copy of list of running transactions.
+    TransactionsList getTransactionsList() const;
+
+private:
+    void loadLogFromZooKeeper();
+    void runUpdatingThread();
+
+    void loadEntries(Strings::const_iterator beg, Strings::const_iterator end);
+    void loadNewEntries();
+    void removeOldEntries();
+
+    static UInt64 deserializeCSN(const String & csn_node_name);
+    static String serializeCSN(CSN csn);
+    static TransactionID deserializeTID(const String & csn_node_content);
+    static String serializeTID(const TransactionID & tid);
+
+    ZooKeeperPtr getZooKeeper() const;
+
+    CSN getCSNImpl(const TIDHash & tid_hash) const;
+
+    ContextPtr global_context;
+    Poco::Logger * log;
+
+    /// The newest snapshot available for reading
+    std::atomic<CSN> latest_snapshot;
+
+    /// Local part of TransactionID number. We reset this counter for each new snapshot.
+    std::atomic<LocalTID> local_tid_counter;
+
+    mutable std::mutex mutex;
+    /// Mapping from TransactionID to CSN for recently committed transactions.
+    /// Allows to check if some transactions is committed.
+    struct CSNEntry
+    {
+        CSN csn;
+        TransactionID tid;
+    };
+    using TIDMap = std::unordered_map<TIDHash, CSNEntry>;
+    TIDMap tid_to_csn;
+
+    mutable std::mutex running_list_mutex;
+    /// Transactions that are currently processed
+    TransactionsList running_list;
+    /// Ordered list of snapshots that are currently used by some transactions. Needed for background cleanup.
+    std::list<CSN> snapshots_in_use;
+
+    ZooKeeperPtr zookeeper;
+    String zookeeper_path;
+
+    String zookeeper_path_log;
+    /// Name of the newest entry that was loaded from log in ZK
+    String last_loaded_entry;
+    /// The oldest CSN such that we store in log entries with TransactionIDs containing this CSN.
+    std::atomic<CSN> tail_ptr = Tx::UnknownCSN;
+
+    zkutil::EventPtr log_updated_event = std::make_shared<Poco::Event>();
+
+    std::atomic_bool stop_flag = false;
+    ThreadFromGlobalPool updating_thread;
+};
+
+template <typename Derived>
+Derived & SingletonHelper<Derived>::createInstanceOrThrow()
+{
+    std::lock_guard lock{instance_mutex};
+    if (!instance_holder)
+    {
+        instance_holder = std::make_shared<Derived>();
+        instance_raw_ptr = instance_holder.get();
+    }
+    return *instance_holder;
+}
+
+}

src/Interpreters/TransactionVersionMetadata.cpp

@@ -0,0 +1,431 @@
+#include <Interpreters/TransactionVersionMetadata.h>
+#include <Interpreters/TransactionsInfoLog.h>
+#include <DataTypes/DataTypesNumber.h>
+#include <DataTypes/DataTypeTuple.h>
+#include <DataTypes/DataTypeUUID.h>
+#include <Interpreters/TransactionLog.h>
+#include <Interpreters/Context.h>
+#include <IO/ReadHelpers.h>
+#include <IO/WriteHelpers.h>
+#include <IO/WriteBufferFromString.h>
+#include <base/logger_useful.h>
+
+namespace DB
+{
+
+namespace ErrorCodes
+{
+    extern const int SERIALIZATION_ERROR;
+    extern const int LOGICAL_ERROR;
+    extern const int CANNOT_PARSE_TEXT;
+}
+
+inline static CSN getCSNAndAssert(TIDHash tid_hash, std::atomic<CSN> & csn, const TransactionID * tid = nullptr)
+{
+    CSN maybe_csn = TransactionLog::getCSN(tid_hash);
+    if (maybe_csn)
+        return maybe_csn;
+
+    /// Either transaction is not committed (yet) or it was committed and then the CSN entry was cleaned up from the log.
+    /// We should load CSN again to distinguish the second case.
+    /// If entry was cleaned up, then CSN is already stored in VersionMetadata and we will get it.
+    /// And for the first case we will get UnknownCSN again.
+    maybe_csn = csn.load();
+    if (maybe_csn)
+        return maybe_csn;
+
+    if (tid)
+        TransactionLog::assertTIDIsNotOutdated(*tid);
+
+    return Tx::UnknownCSN;
+}
+
+VersionMetadata::VersionMetadata()
+{
+    /// It would be better to make it static, but static loggers do not work for some reason (initialization order?)
+    log = &Poco::Logger::get("VersionMetadata");
+}
+
+/// It can be used for introspection purposes only
+TransactionID VersionMetadata::getRemovalTID() const
+{
+    TIDHash removal_lock = removal_tid_lock.load();
+    if (removal_lock)
+    {
+        if (removal_lock == Tx::PrehistoricTID.getHash())
+            return Tx::PrehistoricTID;
+        if (auto txn = TransactionLog::instance().tryGetRunningTransaction(removal_lock))
+            return txn->tid;
+    }
+
+    if (removal_csn.load(std::memory_order_relaxed))
+    {
+        /// removal_tid cannot be changed since we have removal_csn, so it's readonly
+        return removal_tid;
+    }
+
+    return Tx::EmptyTID;
+}
+
+void VersionMetadata::lockRemovalTID(const TransactionID & tid, const TransactionInfoContext & context)
+{
+    LOG_TEST(log, "Trying to lock removal_tid by {}, table: {}, part: {}", tid, context.table.getNameForLogs(), context.part_name);
+    TIDHash locked_by = 0;
+    if (tryLockRemovalTID(tid, context, &locked_by))
+        return;
+
+    String part_desc;
+    if (context.covering_part.empty())
+        part_desc = context.part_name;
+    else
+        part_desc = fmt::format("{} (covered by {})", context.part_name, context.covering_part);
+    throw Exception(ErrorCodes::SERIALIZATION_ERROR,
+                    "Serialization error: "
+                    "Transaction {} tried to remove data part {} from {}, "
+                    "but it's locked by another transaction (TID: {}, TIDH: {}) which is currently removing this part.",
+                    tid, part_desc, context.table.getNameForLogs(), getRemovalTID(), locked_by);
+}
+
+bool VersionMetadata::tryLockRemovalTID(const TransactionID & tid, const TransactionInfoContext & context, TIDHash * locked_by_id)
+{
+    assert(!tid.isEmpty());
+    assert(!creation_tid.isEmpty());
+    TIDHash removal_lock_value = tid.getHash();
+    TIDHash expected_removal_lock_value = 0;
+    bool locked = removal_tid_lock.compare_exchange_strong(expected_removal_lock_value, removal_lock_value);
+    if (!locked)
+    {
+        if (tid == Tx::PrehistoricTID && expected_removal_lock_value == Tx::PrehistoricTID.getHash())
+        {
+            /// Don't need to lock part for queries without transaction
+            LOG_TEST(log, "Assuming removal_tid is locked by {}, table: {}, part: {}", tid, context.table.getNameForLogs(), context.part_name);
+            return true;
+        }
+
+        if (locked_by_id)
+            *locked_by_id = expected_removal_lock_value;
+        return false;
+    }
+
+    removal_tid = tid;
+    tryWriteEventToSystemLog(log, TransactionsInfoLogElement::LOCK_PART, tid, context);
+    return true;
+}
+
+void VersionMetadata::unlockRemovalTID(const TransactionID & tid, const TransactionInfoContext & context)
+{
+    LOG_TEST(log, "Unlocking removal_tid by {}, table: {}, part: {}", tid, context.table.getNameForLogs(), context.part_name);
+    assert(!tid.isEmpty());
+    TIDHash removal_lock_value = tid.getHash();
+    TIDHash locked_by = removal_tid_lock.load();
+
+    auto throw_cannot_unlock = [&]()
+    {
+        auto locked_by_txn = TransactionLog::instance().tryGetRunningTransaction(locked_by);
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "Cannot unlock removal_tid, it's a bug. Current: {} {}, actual: {} {}",
+                        removal_lock_value, tid, locked_by, locked_by_txn ? locked_by_txn->tid : Tx::EmptyTID);
+    };
+
+    if (locked_by != removal_lock_value)
+        throw_cannot_unlock();
+
+    removal_tid = Tx::EmptyTID;
+    bool unlocked = removal_tid_lock.compare_exchange_strong(locked_by, 0);
+    if (!unlocked)
+        throw_cannot_unlock();
+
+    tryWriteEventToSystemLog(log, TransactionsInfoLogElement::UNLOCK_PART, tid, context);
+}
+
+bool VersionMetadata::isRemovalTIDLocked() const
+{
+    return removal_tid_lock.load() != 0;
+}
+
+void VersionMetadata::setCreationTID(const TransactionID & tid, TransactionInfoContext * context)
+{
+    /// NOTE ReplicatedMergeTreeBlockOutputStream may add one part multiple times
+    assert(creation_tid.isEmpty() || creation_tid == tid);
+    creation_tid = tid;
+    if (context)
+        tryWriteEventToSystemLog(log, TransactionsInfoLogElement::ADD_PART, tid, *context);
+}
+
+bool VersionMetadata::isVisible(const MergeTreeTransaction & txn)
+{
+    return isVisible(txn.getSnapshot(), txn.tid);
+}
+
+bool VersionMetadata::isVisible(CSN snapshot_version, TransactionID current_tid)
+{
+    assert(!creation_tid.isEmpty());
+    CSN creation = creation_csn.load(std::memory_order_relaxed);
+    TIDHash removal_lock = removal_tid_lock.load(std::memory_order_relaxed);
+    CSN removal = removal_csn.load(std::memory_order_relaxed);
+
+    [[maybe_unused]] bool had_creation_csn = creation;
+    [[maybe_unused]] bool had_removal_tid = removal_lock;
+    [[maybe_unused]] bool had_removal_csn = removal;
+    assert(!had_removal_csn || had_removal_tid);
+    assert(!had_removal_csn || had_creation_csn);
+    assert(creation == Tx::UnknownCSN || creation == Tx::PrehistoricCSN || Tx::MaxReservedCSN < creation);
+    assert(removal == Tx::UnknownCSN || removal == Tx::PrehistoricCSN || Tx::MaxReservedCSN < removal);
+
+    /// Special snapshot for introspection purposes
+    if (unlikely(snapshot_version == Tx::EverythingVisibleCSN))
+        return true;
+
+    /// Fast path:
+
+    /// Part is definitely not visible if:
+    /// - creation was committed after we took the snapshot
+    /// - removal was committed before we took the snapshot
+    /// - current transaction is removing it
+    if (creation && snapshot_version < creation)
+        return false;
+    if (removal && removal <= snapshot_version)
+        return false;
+    if (!current_tid.isEmpty() && removal_lock && removal_lock == current_tid.getHash())
+        return false;
+
+    /// Otherwise, part is definitely visible if:
+    /// - creation was committed before we took the snapshot and nobody tried to remove the part
+    /// - creation was committed before and removal was committed after
+    /// - current transaction is creating it
+    if (creation && creation <= snapshot_version && !removal_lock)
+        return true;
+    if (creation && creation <= snapshot_version && removal && snapshot_version < removal)
+        return true;
+    if (!current_tid.isEmpty() && creation_tid == current_tid)
+        return true;
+
+    /// End of fast path.
+
+    /// Data part has creation_tid/removal_tid, but does not have creation_csn/removal_csn.
+    /// It means that some transaction is creating/removing the part right now or has done it recently
+    /// and we don't know if it was already committed or not.
+    assert(!had_creation_csn || (had_removal_tid && !had_removal_csn));
+    assert(current_tid.isEmpty() || (creation_tid != current_tid && removal_lock != current_tid.getHash()));
+
+    /// Before doing CSN lookup, let's check some extra conditions.
+    /// If snapshot_version <= some_tid.start_csn, then changes of the transaction with some_tid
+    /// are definitely not visible for us (because the transaction can be committed with greater CSN only),
+    /// so we don't need to check if it was committed.
+    if (snapshot_version <= creation_tid.start_csn)
+        return false;
+
+    /// Check if creation_tid/removal_tid transactions are committed and write CSNs
+    /// TODO Transactions: we probably need more optimizations here
+    /// to avoid some CSN lookups or make the lookups cheaper.
+    /// NOTE: Old enough committed parts always have written CSNs,
+    /// so we can determine their visibility through fast path.
+    /// But for long-running writing transactions we will always do
+    /// CNS lookup and get 0 (UnknownCSN) until the transaction is committed/rolled back.
+    creation = getCSNAndAssert(creation_tid.getHash(), creation_csn, &creation_tid);
+    if (!creation)
+    {
+        return false;   /// Part creation is not committed yet
+    }
+
+    /// We don't need to check if CSNs are already written or not,
+    /// because once written CSN cannot be changed, so it's safe to overwrite it (with the same value).
+    creation_csn.store(creation, std::memory_order_relaxed);
+
+    if (removal_lock)
+    {
+        removal = getCSNAndAssert(removal_lock, removal_csn);
+        if (removal)
+            removal_csn.store(removal, std::memory_order_relaxed);
+    }
+
+    return creation <= snapshot_version && (!removal || snapshot_version < removal);
+}
+
+bool VersionMetadata::canBeRemoved()
+{
+    if (creation_tid == Tx::PrehistoricTID)
+    {
+        /// Avoid access to Transaction log if transactions are not involved
+
+        TIDHash removal_lock = removal_tid_lock.load(std::memory_order_relaxed);
+        if (!removal_lock)
+            return false;
+
+        if (removal_lock == Tx::PrehistoricTID.getHash())
+            return true;
+    }
+
+    return canBeRemovedImpl(TransactionLog::instance().getOldestSnapshot());
+}
+
+bool VersionMetadata::canBeRemovedImpl(CSN oldest_snapshot_version)
+{
+    CSN creation = creation_csn.load(std::memory_order_relaxed);
+    /// We can safely remove part if its creation was rolled back
+    if (creation == Tx::RolledBackCSN)
+        return true;
+
+    if (!creation)
+    {
+        /// Cannot remove part if its creation not committed yet
+        creation = getCSNAndAssert(creation_tid.getHash(), creation_csn, &creation_tid);
+        if (creation)
+            creation_csn.store(creation, std::memory_order_relaxed);
+        else
+            return false;
+    }
+
+    /// Part is probably visible for some transactions (part is too new or the oldest snapshot is too old)
+    if (oldest_snapshot_version < creation)
+        return false;
+
+    TIDHash removal_lock = removal_tid_lock.load(std::memory_order_relaxed);
+    /// Part is active
+    if (!removal_lock)
+        return false;
+
+    CSN removal = removal_csn.load(std::memory_order_relaxed);
+    if (!removal)
+    {
+        /// Part removal is not committed yet
+        removal = getCSNAndAssert(removal_lock, removal_csn);
+        if (removal)
+            removal_csn.store(removal, std::memory_order_relaxed);
+        else
+            return false;
+    }
+
+    /// We can safely remove part if all running transactions were started after part removal was committed
+    return removal <= oldest_snapshot_version;
+}
+
+#define CREATION_TID_STR "creation_tid: "
+#define CREATION_CSN_STR "creation_csn: "
+#define REMOVAL_TID_STR  "removal_tid:  "
+#define REMOVAL_CSN_STR  "removal_csn:  "
+
+
+void VersionMetadata::writeCSN(WriteBuffer & buf, WhichCSN which_csn, bool internal /* = false*/) const
+{
+    if (which_csn == CREATION)
+    {
+        if (CSN creation = creation_csn.load())
+        {
+            writeCString("\n" CREATION_CSN_STR, buf);
+            writeText(creation, buf);
+        }
+        else if (!internal)
+            throw Exception(ErrorCodes::LOGICAL_ERROR, "writeCSN called for creation_csn = 0, it's a bug");
+    }
+    else /// if (which_csn == REMOVAL)
+    {
+        if (CSN removal = removal_csn.load())
+        {
+            writeCString("\n" REMOVAL_CSN_STR, buf);
+            writeText(removal, buf);
+        }
+        else if (!internal)
+            throw Exception(ErrorCodes::LOGICAL_ERROR, "writeCSN called for removal_csn = 0, it's a bug");
+    }
+}
+
+void VersionMetadata::writeRemovalTID(WriteBuffer & buf, bool clear) const
+{
+    writeCString("\n" REMOVAL_TID_STR, buf);
+    if (clear)
+        TransactionID::write(Tx::EmptyTID, buf);
+    else
+        TransactionID::write(removal_tid, buf);
+}
+
+void VersionMetadata::write(WriteBuffer & buf) const
+{
+    writeCString("version: 1", buf);
+    writeCString("\n" CREATION_TID_STR, buf);
+    TransactionID::write(creation_tid, buf);
+    writeCSN(buf, CREATION, /* internal */ true);
+
+    if (removal_tid_lock)
+    {
+        assert(!removal_tid.isEmpty());
+        assert(removal_tid.getHash() == removal_tid_lock);
+        writeRemovalTID(buf);
+        writeCSN(buf, REMOVAL, /* internal */ true);
+    }
+}
+
+void VersionMetadata::read(ReadBuffer & buf)
+{
+    constexpr size_t size = sizeof(CREATION_TID_STR) - 1;
+    static_assert(sizeof(CREATION_CSN_STR) - 1 == size);
+    static_assert(sizeof(REMOVAL_TID_STR) - 1 == size);
+    static_assert(sizeof(REMOVAL_CSN_STR) - 1 == size);
+
+    assertString("version: 1", buf);
+    assertString("\n" CREATION_TID_STR, buf);
+    creation_tid = TransactionID::read(buf);
+    if (buf.eof())
+        return;
+
+    String name;
+    name.resize(size);
+
+    auto read_csn = [&]()
+    {
+        UInt64 val;
+        readText(val, buf);
+        return val;
+    };
+
+    while (!buf.eof())
+    {
+        assertChar('\n', buf);
+        buf.readStrict(name.data(), size);
+
+        if (name == CREATION_CSN_STR)
+        {
+            assert(!creation_csn);
+            creation_csn = read_csn();
+        }
+        else if (name == REMOVAL_TID_STR)
+        {
+            /// NOTE Metadata file may actually contain multiple creation TIDs, we need the last one.
+            removal_tid = TransactionID::read(buf);
+            if (!removal_tid.isEmpty())
+                removal_tid_lock = removal_tid.getHash();
+        }
+        else if (name == REMOVAL_CSN_STR)
+        {
+            if (removal_tid.isEmpty())
+                throw Exception(ErrorCodes::CANNOT_PARSE_TEXT, "Found removal_csn in metadata file, but removal_tid is {}", removal_tid);
+            assert(!removal_csn);
+            removal_csn = read_csn();
+        }
+        else
+        {
+            throw Exception(ErrorCodes::CANNOT_PARSE_TEXT, "Got unexpected content: {}", name);
+        }
+    }
+}
+
+String VersionMetadata::toString(bool one_line) const
+{
+    WriteBufferFromOwnString buf;
+    write(buf);
+    String res = buf.str();
+    if (one_line)
+        std::replace(res.begin(), res.end(), '\n', ' ');
+    return res;
+}
+
+
+DataTypePtr getTransactionIDDataType()
+{
+    DataTypes types;
+    types.push_back(std::make_shared<DataTypeUInt64>());
+    types.push_back(std::make_shared<DataTypeUInt64>());
+    types.push_back(std::make_shared<DataTypeUUID>());
+    return std::make_shared<DataTypeTuple>(std::move(types));
+}
+
+}

src/Interpreters/TransactionVersionMetadata.h

@@ -0,0 +1,81 @@
+#pragma once
+#include <Common/TransactionID.h>
+#include <Interpreters/StorageID.h>
+
+namespace Poco
+{
+class Logger;
+}
+
+namespace DB
+{
+
+/// This structure allows to pass more information about a part that transaction is trying to create/remove.
+/// It's useful for logging and for exception messages.
+struct TransactionInfoContext
+{
+    /// To which table a part belongs
+    StorageID table = StorageID::createEmpty();
+    /// Name of a part that transaction is trying to create/remove
+    String part_name;
+    /// Optional: name of part that covers `part_name` if transaction is trying to remove `part_name`
+    String covering_part;
+
+    TransactionInfoContext(StorageID id, String part) : table(std::move(id)), part_name(std::move(part)) {}
+};
+
+/// This structure contains metadata of an object (currently it's used for data parts in MergeTree only)
+/// that allows to determine when and by which transaction it has been created/removed
+struct VersionMetadata
+{
+    /// ID of transaction that has created/is trying to create this object
+    TransactionID creation_tid = Tx::EmptyTID;
+    /// ID of transaction that has removed/is trying to remove this object
+    TransactionID removal_tid = Tx::EmptyTID;
+
+    /// Hash of removal_tid, used to lock an object for removal
+    std::atomic<TIDHash> removal_tid_lock = 0;
+
+    /// CSN of transaction that has created this object
+    std::atomic<CSN> creation_csn = Tx::UnknownCSN;
+    /// CSN of transaction that has removed this object
+    std::atomic<CSN> removal_csn = Tx::UnknownCSN;
+
+    /// Checks if an object is visible for transaction or not.
+    bool isVisible(const MergeTreeTransaction & txn);
+    bool isVisible(CSN snapshot_version, TransactionID current_tid = Tx::EmptyTID);
+
+    TransactionID getCreationTID() const { return creation_tid; }
+    TransactionID getRemovalTID() const;
+
+    /// Looks an object for removal, throws if it's already locked by concurrent transaction
+    bool tryLockRemovalTID(const TransactionID & tid, const TransactionInfoContext & context, TIDHash * locked_by_id = nullptr);
+    void lockRemovalTID(const TransactionID & tid, const TransactionInfoContext & context);
+    /// Unlocks an object for removal (when transaction is rolling back)
+    void unlockRemovalTID(const TransactionID & tid, const TransactionInfoContext & context);
+
+    bool isRemovalTIDLocked() const;
+
+    /// It can be called only from MergeTreeTransaction or on server startup
+    void setCreationTID(const TransactionID & tid, TransactionInfoContext * context);
+
+    /// Checks if it's safe to remove outdated version of an object
+    bool canBeRemoved();
+    bool canBeRemovedImpl(CSN oldest_snapshot_version);
+
+    void write(WriteBuffer & buf) const;
+    void read(ReadBuffer & buf);
+
+    enum WhichCSN { CREATION, REMOVAL };
+    void writeCSN(WriteBuffer & buf, WhichCSN which_csn, bool internal = false) const;
+    void writeRemovalTID(WriteBuffer & buf, bool clear = false) const;
+
+    String toString(bool one_line = true) const;
+
+    Poco::Logger * log;
+    VersionMetadata();
+};
+
+DataTypePtr getTransactionIDDataType();
+
+}

src/Interpreters/TransactionsInfoLog.cpp

@@ -0,0 +1,111 @@
+#include <Interpreters/TransactionsInfoLog.h>
+#include <Interpreters/TransactionVersionMetadata.h>
+#include <Interpreters/Context.h>
+#include <Common/TransactionID.h>
+#include <Common/CurrentThread.h>
+#include <Core/NamesAndTypes.h>
+#include <DataTypes/DataTypeDateTime64.h>
+#include <DataTypes/DataTypeDate.h>
+#include <DataTypes/DataTypeEnum.h>
+#include <DataTypes/DataTypeFactory.h>
+#include <DataTypes/DataTypeString.h>
+#include <DataTypes/DataTypesNumber.h>
+#include <DataTypes/DataTypeUUID.h>
+#include <base/getThreadId.h>
+
+namespace DB
+{
+
+NamesAndTypesList TransactionsInfoLogElement::getNamesAndTypes()
+{
+    auto type_enum = std::make_shared<DataTypeEnum8>(
+        DataTypeEnum8::Values
+        {
+            {"Begin",           static_cast<Int8>(BEGIN)},
+            {"Commit",          static_cast<Int8>(COMMIT)},
+            {"Rollback",        static_cast<Int8>(ROLLBACK)},
+
+            {"AddPart",         static_cast<Int8>(ADD_PART)},
+            {"LockPart",        static_cast<Int8>(LOCK_PART)},
+            {"UnlockPart",      static_cast<Int8>(UNLOCK_PART)},
+        });
+
+    return
+    {
+        {"type", std::move(type_enum)},
+        {"event_date", std::make_shared<DataTypeDate>()},
+        {"event_time", std::make_shared<DataTypeDateTime64>(6)},
+        {"thread_id", std::make_shared<DataTypeUInt64>()},
+
+        {"query_id", std::make_shared<DataTypeString>()},
+        {"tid", getTransactionIDDataType()},
+        {"tid_hash", std::make_shared<DataTypeUInt64>()},
+
+        {"csn", std::make_shared<DataTypeUInt64>()},
+
+        {"database", std::make_shared<DataTypeString>()},
+        {"table", std::make_shared<DataTypeString>()},
+        {"uuid", std::make_shared<DataTypeUUID>()},
+        {"part", std::make_shared<DataTypeString>()},
+    };
+}
+
+void TransactionsInfoLogElement::fillCommonFields(const TransactionInfoContext * context)
+{
+    event_time = std::chrono::duration_cast<std::chrono::microseconds>(std::chrono::system_clock::now().time_since_epoch()).count();
+    thread_id = getThreadId();
+
+    query_id = CurrentThread::getQueryId().toString();
+
+    if (!context)
+        return;
+
+    table = context->table;
+    part_name = context->part_name;
+}
+
+void TransactionsInfoLogElement::appendToBlock(MutableColumns & columns) const
+{
+    assert(type != UNKNOWN);
+    size_t i = 0;
+
+    columns[i++]->insert(type);
+    auto event_time_seconds = event_time / 1000000;
+    columns[i++]->insert(DateLUT::instance().toDayNum(event_time_seconds).toUnderType());
+    columns[i++]->insert(event_time);
+    columns[i++]->insert(thread_id);
+
+    columns[i++]->insert(query_id);
+    columns[i++]->insert(Tuple{tid.start_csn, tid.local_tid, tid.host_id});
+    columns[i++]->insert(tid.getHash());
+
+    columns[i++]->insert(csn);
+
+    columns[i++]->insert(table.database_name);
+    columns[i++]->insert(table.table_name);
+    columns[i++]->insert(table.uuid);
+    columns[i++]->insert(part_name);
+}
+
+
+void tryWriteEventToSystemLog(Poco::Logger * log,
+                              TransactionsInfoLogElement::Type type, const TransactionID & tid,
+                              const TransactionInfoContext & context)
+try
+{
+    auto system_log =  Context::getGlobalContextInstance()->getTransactionsInfoLog();
+    if (!system_log)
+        return;
+
+    TransactionsInfoLogElement elem;
+    elem.type = type;
+    elem.tid = tid;
+    elem.fillCommonFields(&context);
+    system_log->add(elem);
+}
+catch (...)
+{
+    tryLogCurrentException(log);
+}
+
+}

src/Interpreters/TransactionsInfoLog.h

@@ -0,0 +1,58 @@
+#pragma once
+
+#include <Core/NamesAndAliases.h>
+#include <Interpreters/SystemLog.h>
+#include <Common/TransactionID.h>
+
+namespace DB
+{
+
+struct TransactionInfoContext;
+
+struct TransactionsInfoLogElement
+{
+    enum Type
+    {
+        UNKNOWN = 0,
+
+        BEGIN = 1,
+        COMMIT = 2,
+        ROLLBACK = 3,
+
+        ADD_PART = 10,
+        LOCK_PART = 11,
+        UNLOCK_PART = 12,
+    };
+
+    Type type = UNKNOWN;
+    Decimal64 event_time = 0;
+    UInt64 thread_id;
+
+    String query_id;
+    TransactionID tid = Tx::EmptyTID;
+
+    /// For COMMIT events
+    CSN csn = Tx::UnknownCSN;
+
+    /// For *_PART events
+    StorageID table = StorageID::createEmpty();
+    String part_name;
+
+    static std::string name() { return "TransactionsInfoLog"; }
+    static NamesAndTypesList getNamesAndTypes();
+    static NamesAndAliases getNamesAndAliases() { return {}; }
+    void appendToBlock(MutableColumns & columns) const;
+
+    void fillCommonFields(const TransactionInfoContext * context = nullptr);
+};
+
+class TransactionsInfoLog : public SystemLog<TransactionsInfoLogElement>
+{
+    using SystemLog<TransactionsInfoLogElement>::SystemLog;
+};
+
+
+void tryWriteEventToSystemLog(Poco::Logger * log, TransactionsInfoLogElement::Type type,
+                              const TransactionID & tid, const TransactionInfoContext & context);
+
+}

src/Interpreters/executeDDLQueryOnCluster.cpp

@@ -63,6 +63,9 @@ BlockIO executeDDLQueryOnCluster(const ASTPtr & query_ptr, ContextPtr context, c
 
 BlockIO executeDDLQueryOnCluster(const ASTPtr & query_ptr_, ContextPtr context, AccessRightsElements && query_requires_access)
 {
+    if (context->getCurrentTransaction() && context->getSettingsRef().throw_on_unsupported_query_inside_transaction)
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED, "ON CLUSTER queries inside transactions are not supported");
+
     /// Remove FORMAT <fmt> and INTO OUTFILE <file> if exists
     ASTPtr query_ptr = query_ptr_->clone();
     ASTQueryWithOutput::resetOutputASTIfExist(*query_ptr);

src/Interpreters/executeQuery.cpp

@@ -25,6 +25,8 @@
 #include <Parsers/ASTSelectWithUnionQuery.h>
 #include <Parsers/ASTShowProcesslistQuery.h>
 #include <Parsers/ASTWatchQuery.h>
+#include <Parsers/ASTTransactionControl.h>
+#include <Parsers/ASTExplainQuery.h>
 #include <Parsers/Lexer.h>
 #include <Parsers/parseQuery.h>
 #include <Parsers/ParserQuery.h>
@@ -49,6 +51,7 @@
 #include <Interpreters/ReplaceQueryParameterVisitor.h>
 #include <Interpreters/SelectQueryOptions.h>
 #include <Interpreters/executeQuery.h>
+#include <Interpreters/TransactionLog.h>
 #include <Interpreters/SelectIntersectExceptQueryVisitor.h>
 #include <Common/ProfileEvents.h>
 
@@ -85,6 +88,7 @@ namespace ErrorCodes
 {
     extern const int INTO_OUTFILE_NOT_ALLOWED;
     extern const int QUERY_WAS_CANCELLED;
+    extern const int INVALID_TRANSACTION;
     extern const int LOGICAL_ERROR;
     extern const int NOT_IMPLEMENTED;
 }
@@ -176,10 +180,15 @@ static void logQuery(const String & query, ContextPtr context, bool internal)
         if (!comment.empty())
             comment = fmt::format(" (comment: {})", comment);
 
-        LOG_DEBUG(&Poco::Logger::get("executeQuery"), "(from {}{}{}){} {}",
+        String transaction_info;
+        if (auto txn = context->getCurrentTransaction())
+            transaction_info = fmt::format(" (TID: {}, TIDH: {})", txn->tid, txn->tid.getHash());
+
+        LOG_DEBUG(&Poco::Logger::get("executeQuery"), "(from {}{}{}){}{} {}",
             client_info.current_address.toString(),
             (current_user != "default" ? ", user: " + current_user : ""),
             (!initial_query_id.empty() && current_query_id != initial_query_id ? ", initial_query_id: " + initial_query_id : std::string()),
+            transaction_info,
             comment,
             joinLines(query));
 
@@ -294,6 +303,9 @@ static void onExceptionBeforeStart(const String & query_for_logging, ContextPtr
     if (elem.log_comment.size() > settings.max_query_size)
         elem.log_comment.resize(settings.max_query_size);
 
+    if (auto txn = context->getCurrentTransaction())
+        elem.tid = txn->tid;
+
     if (settings.calculate_text_stack_trace)
         setExceptionStackTrace(elem);
     logException(context, elem);
@@ -428,6 +440,13 @@ static std::tuple<ASTPtr, BlockIO> executeQueryImpl(
         /// TODO: parser should fail early when max_query_size limit is reached.
         ast = parseQuery(parser, begin, end, "", max_query_size, settings.max_parser_depth);
 
+        if (auto txn = context->getCurrentTransaction())
+        {
+            assert(txn->getState() != MergeTreeTransaction::COMMITTED);
+            if (txn->getState() == MergeTreeTransaction::ROLLED_BACK && !ast->as<ASTTransactionControl>() && !ast->as<ASTExplainQuery>())
+                throw Exception(ErrorCodes::INVALID_TRANSACTION, "Cannot execute query: transaction is rolled back");
+        }
+
         /// Interpret SETTINGS clauses as early as possible (before invoking the corresponding interpreter),
         /// to allow settings to take effect.
         if (const auto * select_query = ast->as<ASTSelectQuery>())
@@ -629,11 +648,18 @@ static std::tuple<ASTPtr, BlockIO> executeQueryImpl(
             const auto & table_id = insert_query->table_id;
             if (!table_id.empty())
                 context->setInsertionTable(table_id);
+
+            if (context->getCurrentTransaction() && context->getSettingsRef().throw_on_unsupported_query_inside_transaction)
+                throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Async inserts inside transactions are not supported");
         }
         else
         {
             interpreter = InterpreterFactory::get(ast, context, SelectQueryOptions(stage).setInternal(internal));
 
+            if (context->getCurrentTransaction() && !interpreter->supportsTransactions() &&
+                context->getSettingsRef().throw_on_unsupported_query_inside_transaction)
+                throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Transactions are not supported for this type of query ({})", ast->getID());
+
             if (!interpreter->ignoreQuota())
             {
                 quota = context->getQuota();
@@ -724,6 +750,9 @@ static std::tuple<ASTPtr, BlockIO> executeQueryImpl(
 
             elem.client_info = client_info;
 
+            if (auto txn = context->getCurrentTransaction())
+                elem.tid = txn->tid;
+
             bool log_queries = settings.log_queries && !internal;
 
             /// Log into system table start of query execution, if need.
@@ -945,6 +974,9 @@ static std::tuple<ASTPtr, BlockIO> executeQueryImpl(
                  log_queries_min_query_duration_ms = settings.log_queries_min_query_duration_ms.totalMilliseconds(),
                  quota(quota), status_info_to_query_log] () mutable
             {
+                if (auto txn = context->getCurrentTransaction())
+                    txn->onException();
+
                 if (quota)
                     quota->used(QuotaType::ERRORS, 1, /* check_exceeded = */ false);
 
@@ -1001,6 +1033,9 @@ static std::tuple<ASTPtr, BlockIO> executeQueryImpl(
     }
     catch (...)
     {
+        if (auto txn = context->getCurrentTransaction())
+            txn->onException();
+
         if (!internal)
         {
             if (query_for_logging.empty())

src/Parsers/ASTExplainQuery.h

@@ -19,6 +19,7 @@ public:
         QueryPipeline, /// 'EXPLAIN PIPELINE ...'
         QueryEstimates, /// 'EXPLAIN ESTIMATE ...'
         TableOverride, /// 'EXPLAIN TABLE OVERRIDE ...'
+        CurrentTransaction, /// 'EXPLAIN CURRENT TRANSACTION'
     };
 
     explicit ASTExplainQuery(ExplainKind kind_) : kind(kind_) {}
@@ -111,6 +112,7 @@ private:
             case QueryPipeline: return "EXPLAIN PIPELINE";
             case QueryEstimates: return "EXPLAIN ESTIMATE";
             case TableOverride: return "EXPLAIN TABLE OVERRIDE";
+            case CurrentTransaction: return "EXPLAIN CURRENT TRANSACTION";
         }
 
         __builtin_unreachable();

src/Parsers/ASTKillQueryQuery.cpp

@@ -24,6 +24,9 @@ void ASTKillQueryQuery::formatQueryImpl(const FormatSettings & settings, FormatS
         case Type::PartMoveToShard:
             settings.ostr << "PART_MOVE_TO_SHARD";
             break;
+        case Type::Transaction:
+            settings.ostr << "TRANSACTION";
+            break;
     }
 
     formatOnCluster(settings);

src/Parsers/ASTKillQueryQuery.h

@@ -14,6 +14,7 @@ public:
         Query,      /// KILL QUERY
         Mutation,   /// KILL MUTATION
         PartMoveToShard, /// KILL PART_MOVE_TO_SHARD
+        Transaction,     /// KILL TRANSACTION
     };
 
     Type type = Type::Query;

src/Parsers/ASTTransactionControl.cpp

@@ -0,0 +1,32 @@
+#include <Parsers/ASTTransactionControl.h>
+#include <IO/Operators.h>
+#include <Common/SipHash.h>
+
+namespace DB
+{
+
+void ASTTransactionControl::formatImpl(const FormatSettings & format /*state*/, FormatState &, FormatStateStacked /*frame*/) const
+{
+    switch (action)
+    {
+        case BEGIN:
+            format.ostr << (format.hilite ? hilite_keyword : "") << "BEGIN TRANSACTION" << (format.hilite ? hilite_none : "");
+            break;
+        case COMMIT:
+            format.ostr << (format.hilite ? hilite_keyword : "") << "COMMIT" << (format.hilite ? hilite_none : "");
+            break;
+        case ROLLBACK:
+            format.ostr << (format.hilite ? hilite_keyword : "") << "ROLLBACK" << (format.hilite ? hilite_none : "");
+            break;
+        case SET_SNAPSHOT:
+            format.ostr << (format.hilite ? hilite_keyword : "") << "SET TRANSACTION SNAPSHOT " << (format.hilite ? hilite_none : "") << snapshot;
+            break;
+    }
+}
+
+void ASTTransactionControl::updateTreeHashImpl(SipHash & hash_state) const
+{
+    hash_state.update(action);
+}
+
+}

src/Parsers/ASTTransactionControl.h

@@ -0,0 +1,32 @@
+#pragma once
+#include <Parsers/IAST.h>
+
+namespace DB
+{
+
+/// Common AST for TCL queries
+class ASTTransactionControl : public IAST
+{
+public:
+    enum QueryType
+    {
+        BEGIN,
+        COMMIT,
+        ROLLBACK,
+        SET_SNAPSHOT,
+    };
+
+    QueryType action;
+
+    UInt64 snapshot;    /// For SET TRANSACTION SNAPSHOT ...
+
+    ASTTransactionControl(QueryType action_) : action(action_) {}
+
+    String getID(char /*delimiter*/) const override { return "ASTTransactionControl"; }
+    ASTPtr clone() const override { return std::make_shared<ASTTransactionControl>(*this); }
+
+    void formatImpl(const FormatSettings & format, FormatState & /*state*/, FormatStateStacked /*frame*/) const override;
+    void updateTreeHashImpl(SipHash & hash_state) const override;
+};
+
+}

src/Parsers/ParserExplainQuery.cpp

@@ -22,6 +22,7 @@ bool ParserExplainQuery::parseImpl(Pos & pos, ASTPtr & node, Expected & expected
     ParserKeyword s_plan("PLAN");
     ParserKeyword s_estimates("ESTIMATE");
     ParserKeyword s_table_override("TABLE OVERRIDE");
+    ParserKeyword s_current_transaction("CURRENT TRANSACTION");
 
     if (s_explain.ignore(pos, expected))
     {
@@ -39,6 +40,8 @@ bool ParserExplainQuery::parseImpl(Pos & pos, ASTPtr & node, Expected & expected
             kind = ASTExplainQuery::ExplainKind::QueryEstimates; //-V1048
         else if (s_table_override.ignore(pos, expected))
             kind = ASTExplainQuery::ExplainKind::TableOverride;
+        else if (s_current_transaction.ignore(pos, expected))
+            kind = ASTExplainQuery::ExplainKind::CurrentTransaction;
     }
     else
         return false;
@@ -79,6 +82,10 @@ bool ParserExplainQuery::parseImpl(Pos & pos, ASTPtr & node, Expected & expected
         explain_query->setTableFunction(table_function);
         explain_query->setTableOverride(table_override);
     }
+    else if (kind == ASTExplainQuery::ExplainKind::CurrentTransaction)
+    {
+        /// Nothing to parse
+    }
     else if (select_p.parse(pos, query, expected) ||
         create_p.parse(pos, query, expected) ||
         insert_p.parse(pos, query, expected))

src/Parsers/ParserKillQueryQuery.cpp

@@ -18,6 +18,7 @@ bool ParserKillQueryQuery::parseImpl(Pos & pos, ASTPtr & node, Expected & expect
     ParserKeyword p_query{"QUERY"};
     ParserKeyword p_mutation{"MUTATION"};
     ParserKeyword p_part_move_to_shard{"PART_MOVE_TO_SHARD"};
+    ParserKeyword p_transaction{"TRANSACTION"};
     ParserKeyword p_on{"ON"};
     ParserKeyword p_test{"TEST"};
     ParserKeyword p_sync{"SYNC"};
@@ -34,6 +35,8 @@ bool ParserKillQueryQuery::parseImpl(Pos & pos, ASTPtr & node, Expected & expect
         query->type = ASTKillQueryQuery::Type::Mutation;
     else if (p_part_move_to_shard.ignore(pos, expected))
         query->type = ASTKillQueryQuery::Type::PartMoveToShard;
+    else if (p_transaction.ignore(pos, expected))
+        query->type = ASTKillQueryQuery::Type::Transaction;
     else
         return false;
 

src/Parsers/ParserQuery.cpp

@@ -13,6 +13,7 @@
 #include <Parsers/ParserSystemQuery.h>
 #include <Parsers/ParserUseQuery.h>
 #include <Parsers/ParserExternalDDLQuery.h>
+#include <Parsers/ParserTransactionControl.h>
 
 #include <Parsers/Access/ParserCreateQuotaQuery.h>
 #include <Parsers/Access/ParserCreateRoleQuery.h>
@@ -46,6 +47,7 @@ bool ParserQuery::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
     ParserGrantQuery grant_p;
     ParserSetRoleQuery set_role_p;
     ParserExternalDDLQuery external_ddl_p;
+    ParserTransactionControl transaction_control_p;
     ParserBackupQuery backup_p;
 
     bool res = query_with_output_p.parse(pos, node, expected)
@@ -64,6 +66,7 @@ bool ParserQuery::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
         || drop_access_entity_p.parse(pos, node, expected)
         || grant_p.parse(pos, node, expected)
         || external_ddl_p.parse(pos, node, expected)
+        || transaction_control_p.parse(pos, node, expected)
         || backup_p.parse(pos, node, expected);
 
     return res;

src/Parsers/ParserSetQuery.cpp

@@ -53,6 +53,10 @@ bool ParserSetQuery::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
 
         if (!s_set.ignore(pos, expected))
             return false;
+
+        /// Parse SET TRANSACTION ... queries using ParserTransactionControl
+        if (ParserKeyword{"TRANSACTION"}.check(pos, expected))
+            return false;
     }
 
     SettingsChanges changes;

src/Parsers/ParserTransactionControl.cpp

@@ -0,0 +1,41 @@
+#include <Parsers/ParserTransactionControl.h>
+#include <Parsers/ASTTransactionControl.h>
+#include <Parsers/CommonParsers.h>
+#include <Parsers/ExpressionElementParsers.h>
+#include <Parsers/ASTLiteral.h>
+
+namespace DB
+{
+
+bool ParserTransactionControl::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
+{
+    ASTTransactionControl::QueryType action;
+    UInt64 snapshot = 0;
+
+    if (ParserKeyword("BEGIN TRANSACTION").ignore(pos, expected))
+        action = ASTTransactionControl::BEGIN;
+    else if (ParserKeyword("COMMIT").ignore(pos, expected))
+        action = ASTTransactionControl::COMMIT;
+    else if (ParserKeyword("ROLLBACK").ignore(pos, expected))
+        action = ASTTransactionControl::ROLLBACK;
+    else if (ParserKeyword("SET TRANSACTION SNAPSHOT").ignore(pos, expected))
+    {
+        action = ASTTransactionControl::SET_SNAPSHOT;
+        ASTPtr ast;
+        if (!ParserNumber{}.parse(pos, ast, expected))
+            return false;
+
+        const auto & snapshot_num = ast->as<ASTLiteral>()->value;
+        if (!snapshot_num.tryGet(snapshot))
+            return false;
+    }
+    else
+        return false;
+
+    auto ast = std::make_shared<ASTTransactionControl>(action);
+    ast->snapshot = snapshot;
+    node = ast;
+    return true;
+}
+
+}

src/Parsers/ParserTransactionControl.h

@@ -0,0 +1,14 @@
+#pragma once
+#include <Parsers/IParserBase.h>
+
+namespace DB
+{
+
+class ParserTransactionControl : public IParserBase
+{
+public:
+    const char * getName() const override { return "TCL query"; }
+    bool parseImpl(Pos & pos, ASTPtr & node, Expected & expected) override;
+};
+
+}

src/QueryPipeline/RemoteQueryExecutor.cpp

@@ -563,7 +563,7 @@ void RemoteQueryExecutor::sendExternalTables()
                 {
                     SelectQueryInfo query_info;
                     auto metadata_snapshot = cur->getInMemoryMetadataPtr();
-                    auto storage_snapshot = cur->getStorageSnapshot(metadata_snapshot);
+                    auto storage_snapshot = cur->getStorageSnapshot(metadata_snapshot, context);
                     QueryProcessingStage::Enum read_from_table_stage = cur->getQueryProcessingStage(
                         context, QueryProcessingStage::Complete, storage_snapshot, query_info);
 

src/Storages/FileLog/StorageFileLog.cpp

@@ -679,7 +679,7 @@ bool StorageFileLog::streamToViews()
         throw Exception("Engine table " + table_id.getNameForLogs() + " doesn't exist", ErrorCodes::LOGICAL_ERROR);
 
     auto metadata_snapshot = getInMemoryMetadataPtr();
-    auto storage_snapshot = getStorageSnapshot(metadata_snapshot);
+    auto storage_snapshot = getStorageSnapshot(metadata_snapshot, getContext());
 
     auto max_streams_number = std::min<UInt64>(filelog_settings->max_threads.value, file_infos.file_names.size());
     /// No files to parse

src/Storages/IStorage.h

@@ -161,6 +161,11 @@ public:
     /// Returns true if the storage supports reading of subcolumns of complex types.
     virtual bool supportsSubcolumns() const { return false; }
 
+    /// Returns true if the storage supports transactions for SELECT, INSERT and ALTER queries.
+    /// Storage may throw an exception later if some query kind is not fully supported.
+    /// This method can return true for readonly engines that return the same rows for reading (such as SystemNumbers)
+    virtual bool supportsTransactions() const { return false; }
+
     /// Returns true if the storage supports storing of dynamic subcolumns.
     /// For now it makes sense only for data type Object.
     virtual bool supportsDynamicSubcolumns() const { return false; }
@@ -483,6 +488,16 @@ public:
         throw Exception("Mutations are not supported by storage " + getName(), ErrorCodes::NOT_IMPLEMENTED);
     }
 
+    virtual void waitForMutation(const String & /*mutation_id*/)
+    {
+        throw Exception("Mutations are not supported by storage " + getName(), ErrorCodes::NOT_IMPLEMENTED);
+    }
+
+    virtual void setMutationCSN(const String & /*mutation_id*/, UInt64 /*csn*/)
+    {
+        throw Exception("Mutations are not supported by storage " + getName(), ErrorCodes::NOT_IMPLEMENTED);
+    }
+
     /// Cancel a part move to shard.
     virtual CancellationCode killPartMoveToShard(const UUID & /*task_uuid*/)
     {
@@ -553,11 +568,6 @@ public:
     /// Similar to above but checks for DETACH. It's only used for DICTIONARIES.
     virtual void checkTableCanBeDetached() const {}
 
-    /// Checks that Partition could be dropped right now
-    /// Otherwise - throws an exception with detailed information.
-    /// We do not use mutex because it is not very important that the size could change during the operation.
-    virtual void checkPartitionCanBeDropped(const ASTPtr & /*partition*/) {}
-
     /// Returns true if Storage may store some data on disk.
     /// NOTE: may not be equivalent to !getDataPaths().empty()
     virtual bool storesDataOnDisk() const { return false; }
@@ -610,15 +620,15 @@ public:
     virtual std::optional<UInt64> lifetimeBytes() const { return {}; }
 
     /// Creates a storage snapshot from given metadata.
-    virtual StorageSnapshotPtr getStorageSnapshot(const StorageMetadataPtr & metadata_snapshot) const
+    virtual StorageSnapshotPtr getStorageSnapshot(const StorageMetadataPtr & metadata_snapshot, ContextPtr /*query_context*/) const
     {
         return std::make_shared<StorageSnapshot>(*this, metadata_snapshot);
     }
 
     /// Creates a storage snapshot from given metadata and columns, which are used in query.
-    virtual StorageSnapshotPtr getStorageSnapshotForQuery(const StorageMetadataPtr & metadata_snapshot, const ASTPtr & /*query*/) const
+    virtual StorageSnapshotPtr getStorageSnapshotForQuery(const StorageMetadataPtr & metadata_snapshot, const ASTPtr & /*query*/, ContextPtr query_context) const
     {
-        return getStorageSnapshot(metadata_snapshot);
+        return getStorageSnapshot(metadata_snapshot, query_context);
     }
 
 private:

src/Storages/Kafka/StorageKafka.cpp

@@ -647,7 +647,7 @@ bool StorageKafka::streamToViews()
     CurrentMetrics::Increment metric_increment{CurrentMetrics::KafkaBackgroundReads};
     ProfileEvents::increment(ProfileEvents::KafkaBackgroundReads);
 
-    auto storage_snapshot = getStorageSnapshot(getInMemoryMetadataPtr());
+    auto storage_snapshot = getStorageSnapshot(getInMemoryMetadataPtr(), getContext());
 
     // Create an INSERT query for streaming data
     auto insert = std::make_shared<ASTInsertQuery>();

src/Storages/MergeTree/DataPartsExchange.cpp

@@ -573,6 +573,7 @@ MergeTreeData::MutableDataPartPtr Fetcher::downloadPartToMemory(
     auto volume = std::make_shared<SingleDiskVolume>("volume_" + part_name, disk, 0);
     MergeTreeData::MutableDataPartPtr new_data_part =
         std::make_shared<MergeTreeDataPartInMemory>(data, part_name, volume);
+    new_data_part->version.setCreationTID(Tx::PrehistoricTID, nullptr);
 
     for (auto i = 0ul; i < projections; ++i)
     {
@@ -601,7 +602,8 @@ MergeTreeData::MutableDataPartPtr Fetcher::downloadPartToMemory(
             metadata_snapshot->projections.get(projection_name).metadata,
             block.getNamesAndTypesList(),
             {},
-            CompressionCodecFactory::instance().get("NONE", {}));
+            CompressionCodecFactory::instance().get("NONE", {}),
+            NO_TRANSACTION_PTR);
 
         part_out.write(block);
         part_out.finalizePart(new_projection_part, false);
@@ -625,7 +627,7 @@ MergeTreeData::MutableDataPartPtr Fetcher::downloadPartToMemory(
 
     MergedBlockOutputStream part_out(
         new_data_part, metadata_snapshot, block.getNamesAndTypesList(), {},
-        CompressionCodecFactory::instance().get("NONE", {}));
+        CompressionCodecFactory::instance().get("NONE", {}), NO_TRANSACTION_PTR);
 
     part_out.write(block);
     part_out.finalizePart(new_data_part, false);
@@ -753,6 +755,7 @@ MergeTreeData::MutableDataPartPtr Fetcher::downloadPartToDisk(
     assertEOF(in);
     auto volume = std::make_shared<SingleDiskVolume>("volume_" + part_name, disk, 0);
     MergeTreeData::MutableDataPartPtr new_data_part = data.createPart(part_name, volume, part_relative_path);
+    new_data_part->version.setCreationTID(Tx::PrehistoricTID, nullptr);
     new_data_part->is_temp = true;
     new_data_part->modification_time = time(nullptr);
     new_data_part->loadColumnsChecksumsIndexes(true, false);
@@ -842,6 +845,7 @@ MergeTreeData::MutableDataPartPtr Fetcher::downloadPartToDiskRemoteMeta(
     assertEOF(in);
 
     MergeTreeData::MutableDataPartPtr new_data_part = data.createPart(part_name, volume, part_relative_path);
+    new_data_part->version.setCreationTID(Tx::PrehistoricTID, nullptr);
     new_data_part->is_temp = true;
     new_data_part->modification_time = time(nullptr);
     new_data_part->loadColumnsChecksumsIndexes(true, false);

src/Storages/MergeTree/IMergeTreeDataPart.cpp

@@ -17,7 +17,6 @@
 #include <Storages/MergeTree/PartMetadataManagerWithCache.h>
 #include <Common/StringUtils/StringUtils.h>
 #include <Common/escapeForFileName.h>
-#include <Common/ZooKeeper/ZooKeeper.h>
 #include <Common/CurrentMetrics.h>
 #include <Common/FieldVisitorsAccurateComparison.h>
 #include <Common/MemoryTrackerBlockerInThread.h>
@@ -29,6 +28,8 @@
 #include <Parsers/ExpressionElementParsers.h>
 #include <DataTypes/NestedUtils.h>
 #include <DataTypes/DataTypeAggregateFunction.h>
+#include <Interpreters/MergeTreeTransaction.h>
+#include <Interpreters/TransactionLog.h>
 
 
 namespace CurrentMetrics
@@ -65,6 +66,12 @@ namespace ErrorCodes
     extern const int NOT_IMPLEMENTED;
 }
 
+static std::unique_ptr<ReadBufferFromFileBase> openForReading(const DiskPtr & disk, const String & path)
+{
+    size_t file_size = disk->getFileSize(path);
+    return disk->readFile(path, ReadSettings().adjustBufferSize(file_size), file_size);
+}
+
 void IMergeTreeDataPart::MinMaxIndex::load(const MergeTreeData & data, const PartMetadataManagerPtr & manager)
 {
     auto metadata_snapshot = data.getInMemoryMetadataPtr();
@@ -466,6 +473,7 @@ SerializationPtr IMergeTreeDataPart::getSerialization(const NameAndTypePair & co
 
 void IMergeTreeDataPart::removeIfNeeded()
 {
+    assert(assertHasValidVersionMetadata());
     if (!is_temp && state != State::DeleteOnDestroy)
         return;
 
@@ -790,10 +798,14 @@ NameSet IMergeTreeDataPart::getFileNamesWithoutChecksums() const
 
     NameSet result = {"checksums.txt", "columns.txt"};
     String default_codec_path = fs::path(getFullRelativePath()) / DEFAULT_COMPRESSION_CODEC_FILE_NAME;
+    String txn_version_path = fs::path(getFullRelativePath()) / TXN_VERSION_METADATA_FILE_NAME;
 
     if (volume->getDisk()->exists(default_codec_path))
         result.emplace(DEFAULT_COMPRESSION_CODEC_FILE_NAME);
 
+    if (volume->getDisk()->exists(txn_version_path))
+        result.emplace(TXN_VERSION_METADATA_FILE_NAME);
+
     return result;
 }
 
@@ -1223,6 +1235,218 @@ void IMergeTreeDataPart::loadColumns(bool require)
     setSerializationInfos(infos);
 }
 
+void IMergeTreeDataPart::assertHasVersionMetadata(MergeTreeTransaction * txn) const
+{
+    TransactionID expected_tid = txn ? txn->tid : Tx::PrehistoricTID;
+    if (version.creation_tid != expected_tid)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+                        "CreationTID of part {} (table {}) is set to unexpected value {}, it's a bug. Current transaction: {}",
+                        name, storage.getStorageID().getNameForLogs(), version.creation_tid, txn ? txn->dumpDescription() : "<none>");
+
+    assert(!txn || storage.supportsTransactions());
+    assert(!txn || volume->getDisk()->exists(fs::path(getFullRelativePath()) / TXN_VERSION_METADATA_FILE_NAME));
+}
+
+void IMergeTreeDataPart::storeVersionMetadata() const
+{
+    if (!wasInvolvedInTransaction())
+        return;
+
+    LOG_TEST(storage.log, "Writing version for {} (creation: {}, removal {})", name, version.creation_tid, version.removal_tid);
+    assert(storage.supportsTransactions());
+
+    if (!isStoredOnDisk())
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Transactions are not supported for in-memory parts (table: {}, part: {})",
+                        storage.getStorageID().getNameForLogs(), name);
+
+    String version_file_name = fs::path(getFullRelativePath()) / TXN_VERSION_METADATA_FILE_NAME;
+    String tmp_version_file_name = version_file_name + ".tmp";
+    DiskPtr disk = volume->getDisk();
+    {
+        /// TODO IDisk interface does not allow to open file with O_EXCL flag (for DiskLocal),
+        /// so we create empty file at first (expecting that createFile throws if file already exists)
+        /// and then overwrite it.
+        disk->createFile(tmp_version_file_name);
+        auto out = disk->writeFile(tmp_version_file_name, 256, WriteMode::Rewrite);
+        version.write(*out);
+        out->finalize();
+        out->sync();
+    }
+
+    SyncGuardPtr sync_guard;
+    if (storage.getSettings()->fsync_part_directory)
+        sync_guard = disk->getDirectorySyncGuard(getFullRelativePath());
+    disk->replaceFile(tmp_version_file_name, version_file_name);
+}
+
+void IMergeTreeDataPart::appendCSNToVersionMetadata(VersionMetadata::WhichCSN which_csn) const
+{
+    assert(!version.creation_tid.isEmpty());
+    assert(!(which_csn == VersionMetadata::WhichCSN::CREATION && version.creation_tid.isPrehistoric()));
+    assert(!(which_csn == VersionMetadata::WhichCSN::CREATION && version.creation_csn == 0));
+    assert(!(which_csn == VersionMetadata::WhichCSN::REMOVAL && (version.removal_tid.isPrehistoric() || version.removal_tid.isEmpty())));
+    assert(!(which_csn == VersionMetadata::WhichCSN::REMOVAL && version.removal_csn == 0));
+    assert(isStoredOnDisk());
+
+    /// Small enough appends to file are usually atomic,
+    /// so we append new metadata instead of rewriting file to reduce number of fsyncs.
+    /// We don't need to do fsync when writing CSN, because in case of hard restart
+    /// we will be able to restore CSN from transaction log in Keeper.
+
+    String version_file_name = fs::path(getFullRelativePath()) / TXN_VERSION_METADATA_FILE_NAME;
+    DiskPtr disk = volume->getDisk();
+    auto out = disk->writeFile(version_file_name, 256, WriteMode::Append);
+    version.writeCSN(*out, which_csn);
+    out->finalize();
+}
+
+void IMergeTreeDataPart::appendRemovalTIDToVersionMetadata(bool clear) const
+{
+    assert(!version.creation_tid.isEmpty());
+    assert(version.removal_csn == 0);
+    assert(!version.removal_tid.isEmpty());
+    assert(isStoredOnDisk());
+
+    if (version.creation_tid.isPrehistoric() && !clear)
+    {
+        /// Metadata file probably does not exist, because it was not written on part creation, because it was created without a transaction.
+        /// Let's create it (if needed). Concurrent writes are not possible, because creation_csn is prehistoric and we own removal_tid_lock.
+        storeVersionMetadata();
+        return;
+    }
+
+    if (clear)
+        LOG_TEST(storage.log, "Clearing removal TID for {} (creation: {}, removal {})", name, version.creation_tid, version.removal_tid);
+    else
+        LOG_TEST(storage.log, "Appending removal TID for {} (creation: {}, removal {})", name, version.creation_tid, version.removal_tid);
+
+    String version_file_name = fs::path(getFullRelativePath()) / TXN_VERSION_METADATA_FILE_NAME;
+    DiskPtr disk = volume->getDisk();
+    auto out = disk->writeFile(version_file_name, 256, WriteMode::Append);
+    version.writeRemovalTID(*out, clear);
+    out->finalize();
+
+    /// fsync is not required when we clearing removal TID, because after hard restart we will fix metadata
+    if (!clear)
+        out->sync();
+}
+
+void IMergeTreeDataPart::loadVersionMetadata() const
+try
+{
+    String version_file_name = fs::path(getFullRelativePath()) / TXN_VERSION_METADATA_FILE_NAME;
+    String tmp_version_file_name = version_file_name + ".tmp";
+    DiskPtr disk = volume->getDisk();
+
+    auto remove_tmp_file = [&]()
+    {
+        auto last_modified = disk->getLastModified(tmp_version_file_name);
+        auto buf = openForReading(disk, tmp_version_file_name);
+        String content;
+        readStringUntilEOF(content, *buf);
+        LOG_WARNING(storage.log, "Found file {} that was last modified on {}, has size {} and the following content: {}",
+                    tmp_version_file_name, last_modified.epochTime(), content.size(), content);
+        disk->removeFile(tmp_version_file_name);
+    };
+
+    if (disk->exists(version_file_name))
+    {
+        auto buf = openForReading(disk, version_file_name);
+        version.read(*buf);
+        if (disk->exists(tmp_version_file_name))
+            remove_tmp_file();
+        return;
+    }
+
+    /// Four (?) cases are possible:
+    /// 1. Part was created without transactions.
+    /// 2. Version metadata file was not renamed from *.tmp on part creation.
+    /// 3. Version metadata were written to *.tmp file, but hard restart happened before fsync.
+    /// 4. Fsyncs in storeVersionMetadata() work incorrectly.
+
+    if (!disk->exists(tmp_version_file_name))
+    {
+        /// Case 1.
+        /// We do not have version metadata and transactions history for old parts,
+        /// so let's consider that such parts were created by some ancient transaction
+        /// and were committed with some prehistoric CSN.
+        /// NOTE It might be Case 3, but version metadata file is written on part creation before other files,
+        /// so it's not Case 3 if part is not broken.
+        version.setCreationTID(Tx::PrehistoricTID, nullptr);
+        version.creation_csn = Tx::PrehistoricCSN;
+        return;
+    }
+
+    /// Case 2.
+    /// Content of *.tmp file may be broken, just use fake TID.
+    /// Transaction was not committed if *.tmp file was not renamed, so we should complete rollback by removing part.
+    version.setCreationTID(Tx::DummyTID, nullptr);
+    version.creation_csn = Tx::RolledBackCSN;
+    remove_tmp_file();
+}
+catch (Exception & e)
+{
+    e.addMessage("While loading version metadata from table {} part {}", storage.getStorageID().getNameForLogs(), name);
+    throw;
+}
+
+bool IMergeTreeDataPart::wasInvolvedInTransaction() const
+{
+    assert(!version.creation_tid.isEmpty() || (state == State::Temporary /* && std::uncaught_exceptions() */));
+    bool created_by_transaction = !version.creation_tid.isPrehistoric();
+    bool removed_by_transaction = version.isRemovalTIDLocked() && version.removal_tid_lock != Tx::PrehistoricTID.getHash();
+    return created_by_transaction || removed_by_transaction;
+}
+
+bool IMergeTreeDataPart::assertHasValidVersionMetadata() const
+{
+    /// We don't have many tests with server restarts and it's really inconvenient to write such tests.
+    /// So we use debug assertions to ensure that part version is written correctly.
+    /// This method is not supposed to be called in release builds.
+
+    if (isProjectionPart())
+        return true;
+
+    if (!wasInvolvedInTransaction())
+        return true;
+
+    if (!isStoredOnDisk())
+        return false;
+
+    if (part_is_probably_removed_from_disk)
+        return true;
+
+    DiskPtr disk = volume->getDisk();
+    if (!disk->exists(getFullRelativePath()))
+        return true;
+
+    String content;
+    String version_file_name = fs::path(getFullRelativePath()) / TXN_VERSION_METADATA_FILE_NAME;
+    try
+    {
+        auto buf = openForReading(disk, version_file_name);
+        readStringUntilEOF(content, *buf);
+        ReadBufferFromString str_buf{content};
+        VersionMetadata file;
+        file.read(str_buf);
+        bool valid_creation_tid = version.creation_tid == file.creation_tid;
+        bool valid_removal_tid = version.removal_tid == file.removal_tid || version.removal_tid == Tx::PrehistoricTID;
+        bool valid_creation_csn = version.creation_csn == file.creation_csn || version.creation_csn == Tx::RolledBackCSN;
+        bool valid_removal_csn = version.removal_csn == file.removal_csn || version.removal_csn == Tx::PrehistoricCSN;
+        if (!valid_creation_tid || !valid_removal_tid || !valid_creation_csn || !valid_removal_csn)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "Invalid version metadata file");
+        return true;
+    }
+    catch (...)
+    {
+        WriteBufferFromOwnString expected;
+        version.write(expected);
+        tryLogCurrentException(storage.log, fmt::format("File {} contains:\n{}\nexpected:\n{}", version_file_name, content, expected.str()));
+        return false;
+    }
+}
+
+
 void IMergeTreeDataPart::appendFilesOfColumns(Strings & files)
 {
     files.push_back("columns.txt");
@@ -1337,6 +1561,9 @@ void IMergeTreeDataPart::initializePartMetadataManager()
 
 void IMergeTreeDataPart::remove() const
 {
+    assert(assertHasValidVersionMetadata());
+    part_is_probably_removed_from_disk = true;
+
     std::optional<bool> keep_shared_data = keepSharedDataInDecoupledStorage();
     if (!keep_shared_data.has_value())
         return;
@@ -1444,6 +1671,7 @@ void IMergeTreeDataPart::remove() const
 
             request.emplace_back(fs::path(to) / DEFAULT_COMPRESSION_CODEC_FILE_NAME, true);
             request.emplace_back(fs::path(to) / DELETE_ON_DESTROY_MARKER_FILE_NAME, true);
+            request.emplace_back(fs::path(to) / TXN_VERSION_METADATA_FILE_NAME, true);
 
             disk->removeSharedFiles(request, *keep_shared_data);
             disk->removeDirectory(to);

src/Storages/MergeTree/IMergeTreeDataPart.h

@@ -13,6 +13,7 @@
 #include <Storages/MergeTree/MergeTreeDataPartTTLInfo.h>
 #include <Storages/MergeTree/MergeTreeIOSettings.h>
 #include <Storages/MergeTree/KeyCondition.h>
+#include <Interpreters/TransactionVersionMetadata.h>
 #include <DataTypes/Serializations/SerializationInfo.h>
 #include <Storages/MergeTree/IPartMetadataManager.h>
 
@@ -40,6 +41,7 @@ class IMergeTreeReader;
 class IMergeTreeDataPartWriter;
 class MarkCache;
 class UncompressedCache;
+class MergeTreeTransaction;
 
 /// Description of the data part.
 class IMergeTreeDataPart : public std::enable_shared_from_this<IMergeTreeDataPart>
@@ -327,6 +329,8 @@ public:
 
     CompressionCodecPtr default_codec;
 
+    mutable VersionMetadata version;
+
     /// For data in RAM ('index')
     UInt64 getIndexSizeInBytes() const;
     UInt64 getIndexSizeInAllocatedBytes() const;
@@ -414,6 +418,8 @@ public:
     /// (number of rows, number of rows with default values, etc).
     static inline constexpr auto SERIALIZATION_FILE_NAME = "serialization.json";
 
+    static inline constexpr auto TXN_VERSION_METADATA_FILE_NAME = "txn_version.txt";
+
     /// One of part files which is used to check how many references (I'd like
     /// to say hardlinks, but it will confuse even more) we have for the part
     /// for zero copy replication. Sadly it's very complex.
@@ -435,12 +441,38 @@ public:
     /// Required for distinguish different copies of the same part on remote FS.
     String getUniqueId() const;
 
+    /// Ensures that creation_tid was correctly set after part creation.
+    void assertHasVersionMetadata(MergeTreeTransaction * txn) const;
+
+    /// [Re]writes file with transactional metadata on disk
+    void storeVersionMetadata() const;
+
+    /// Appends the corresponding CSN to file on disk (without fsync)
+    void appendCSNToVersionMetadata(VersionMetadata::WhichCSN which_csn) const;
+
+    /// Appends removal TID to file on disk (with fsync)
+    void appendRemovalTIDToVersionMetadata(bool clear = false) const;
+
+    /// Loads transactional metadata from disk
+    void loadVersionMetadata() const;
+
+    /// Returns true if part was created or removed by a transaction
+    bool wasInvolvedInTransaction() const;
+
+    /// Moar hardening: this method is supposed to be used for debug assertions
+    bool assertHasValidVersionMetadata() const;
+
+    /// Return hardlink count for part.
+    /// Required for keep data on remote FS when part has shadow copies.
+    UInt32 getNumberOfRefereneces() const;
+
     /// Get checksums of metadata file in part directory
     IMergeTreeDataPart::uint128 getActualChecksumByFile(const String & file_path) const;
 
     /// Check metadata in cache is consistent with actual metadata on disk(if use_metadata_cache is true)
     std::unordered_map<String, uint128> checkMetadata() const;
 
+
 protected:
 
     /// Total size of all columns, calculated once in calcuateColumnSizesOnDisk
@@ -551,6 +583,9 @@ private:
     CompressionCodecPtr detectDefaultCompressionCodec() const;
 
     mutable State state{State::Temporary};
+
+    /// This ugly flag is needed for debug assertions only
+    mutable bool part_is_probably_removed_from_disk = false;
 };
 
 using MergeTreeDataPartState = IMergeTreeDataPart::State;

src/Storages/MergeTree/MergeFromLogEntryTask.cpp

@@ -227,7 +227,7 @@ ReplicatedMergeMutateTaskBase::PrepareResult MergeFromLogEntryTask::prepare()
         future_merged_part,
         settings);
 
-    transaction_ptr = std::make_unique<MergeTreeData::Transaction>(storage);
+    transaction_ptr = std::make_unique<MergeTreeData::Transaction>(storage, NO_TRANSACTION_RAW);
     stopwatch_ptr = std::make_unique<Stopwatch>();
 
     merge_task = storage.merger_mutator.mergePartsToTemporaryPart(
@@ -241,7 +241,8 @@ ReplicatedMergeMutateTaskBase::PrepareResult MergeFromLogEntryTask::prepare()
             reserved_space,
             entry.deduplicate,
             entry.deduplicate_by_columns,
-            storage.merging_params);
+            storage.merging_params,
+            NO_TRANSACTION_PTR);
 
 
     /// Adjust priority
@@ -264,7 +265,7 @@ bool MergeFromLogEntryTask::finalize(ReplicatedMergeMutateTaskBase::PartLogWrite
     /// Task is not needed
     merge_task.reset();
 
-    storage.merger_mutator.renameMergedTemporaryPart(part, parts, transaction_ptr.get());
+    storage.merger_mutator.renameMergedTemporaryPart(part, parts, NO_TRANSACTION_PTR, transaction_ptr.get());
 
     try
     {

src/Storages/MergeTree/MergeMutateSelectedEntry.h

@@ -39,10 +39,13 @@ struct MergeMutateSelectedEntry
     FutureMergedMutatedPartPtr future_part;
     CurrentlyMergingPartsTaggerPtr tagger;
     MutationCommandsConstPtr commands;
-    MergeMutateSelectedEntry(FutureMergedMutatedPartPtr future_part_, CurrentlyMergingPartsTaggerPtr tagger_, MutationCommandsConstPtr commands_)
+    MergeTreeTransactionPtr txn;
+    MergeMutateSelectedEntry(FutureMergedMutatedPartPtr future_part_, CurrentlyMergingPartsTaggerPtr tagger_,
+                             MutationCommandsConstPtr commands_, const MergeTreeTransactionPtr & txn_ = NO_TRANSACTION_PTR)
         : future_part(future_part_)
         , tagger(std::move(tagger_))
         , commands(commands_)
+        , txn(txn_)
     {}
 };
 

src/Storages/MergeTree/MergePlainMergeTreeTask.cpp

@@ -107,14 +107,15 @@ void MergePlainMergeTreeTask::prepare()
             merge_mutate_entry->tagger->reserved_space,
             deduplicate,
             deduplicate_by_columns,
-            storage.merging_params);
+            storage.merging_params,
+            txn);
 }
 
 
 void MergePlainMergeTreeTask::finish()
 {
     new_part = merge_task->getFuture().get();
-    storage.merger_mutator.renameMergedTemporaryPart(new_part, future_part->parts, nullptr);
+    storage.merger_mutator.renameMergedTemporaryPart(new_part, future_part->parts, txn, nullptr);
     write_part_log({});
 }
 

src/Storages/MergeTree/MergePlainMergeTreeTask.h

@@ -39,6 +39,12 @@ public:
     StorageID getStorageID() override;
     UInt64 getPriority() override { return priority; }
 
+    void setCurrentTransaction(MergeTreeTransactionHolder && txn_holder_, MergeTreeTransactionPtr && txn_)
+    {
+        txn_holder = std::move(txn_holder_);
+        txn = std::move(txn_);
+    }
+
 private:
 
     void prepare();
@@ -73,6 +79,9 @@ private:
     std::function<void(const ExecutionStatus &)> write_part_log;
     IExecutableTask::TaskResultCallback task_result_callback;
     MergeTaskPtr merge_task{nullptr};
+
+    MergeTreeTransactionHolder txn_holder;
+    MergeTreeTransactionPtr txn;
 };
 
 

src/Storages/MergeTree/MergeTask.cpp

@@ -260,6 +260,7 @@ bool MergeTask::ExecuteAndFinalizeHorizontalPart::prepare()
         global_ctx->merging_columns,
         MergeTreeIndexFactory::instance().getMany(global_ctx->metadata_snapshot->getSecondaryIndices()),
         ctx->compression_codec,
+        global_ctx->txn,
         /*reset_columns=*/ true,
         ctx->blocks_are_granules_size,
         global_ctx->context->getWriteSettings());
@@ -593,6 +594,7 @@ bool MergeTask::MergeProjectionsStage::mergeMinMaxIndexAndPrepareProjections() c
             projection_merging_params,
             global_ctx->new_data_part.get(),
             ".proj",
+            NO_TRANSACTION_PTR,
             global_ctx->data,
             global_ctx->mutator,
             global_ctx->merges_blocker,

src/Storages/MergeTree/MergeTask.h

@@ -60,6 +60,7 @@ public:
         MergeTreeData::MergingParams merging_params_,
         const IMergeTreeDataPart * parent_part_,
         String suffix_,
+        MergeTreeTransactionPtr txn,
         MergeTreeData * data_,
         MergeTreeDataMergerMutator * mutator_,
         ActionBlocker * merges_blocker_,
@@ -83,6 +84,7 @@ public:
             global_ctx->mutator = std::move(mutator_);
             global_ctx->merges_blocker = std::move(merges_blocker_);
             global_ctx->ttl_merges_blocker = std::move(ttl_merges_blocker_);
+            global_ctx->txn = std::move(txn);
 
             auto prepare_stage_ctx = std::make_shared<ExecuteAndFinalizeHorizontalPartRuntimeContext>();
 
@@ -164,6 +166,8 @@ private:
         std::promise<MergeTreeData::MutableDataPartPtr> promise{};
 
         IMergedBlockOutputStream::WrittenOffsetColumns written_offset_columns{};
+
+        MergeTreeTransactionPtr txn;
     };
 
     using GlobalRuntimeContextPtr = std::shared_ptr<GlobalRuntimeContext>;

src/Storages/MergeTree/MergeTreeData.cpp

@@ -30,8 +30,10 @@
 #include <Interpreters/PartLog.h>
 #include <Interpreters/TreeRewriter.h>
 #include <Interpreters/inplaceBlockConversions.h>
+#include <Interpreters/MergeTreeTransaction.h>
 #include <Interpreters/Context.h>
 #include <Interpreters/InterpreterSelectQuery.h>
+#include <Interpreters/TransactionLog.h>
 #include <Interpreters/evaluateConstantExpression.h>
 #include <Interpreters/convertFieldToType.h>
 #include <Parsers/ASTFunction.h>
@@ -49,8 +51,6 @@
 #include <Storages/MergeTree/MergeTreeDataPartInMemory.h>
 #include <Storages/MergeTree/MergeTreeDataPartWide.h>
 #include <Storages/MergeTree/MergeTreeSequentialSource.h>
-#include <Storages/MergeTree/MergedBlockOutputStream.h>
-#include <Storages/MergeTree/MergedColumnOnlyOutputStream.h>
 #include <Storages/MergeTree/checkDataPart.h>
 #include <Storages/MergeTree/localBackup.h>
 #include <Storages/StorageMergeTree.h>
@@ -1320,51 +1320,155 @@ void MergeTreeData::loadDataParts(bool skip_sanity_checks)
     for (auto & part : duplicate_parts_to_remove)
         part->remove();
 
-    /// Delete from the set of current parts those parts that are covered by another part (those parts that
-    /// were merged), but that for some reason are still not deleted from the filesystem.
-    /// Deletion of files will be performed later in the clearOldParts() method.
-
-    if (data_parts_indexes.size() >= 2)
-    {
-        /// Now all parts are committed, so data_parts_by_state_and_info == committed_parts_range
-        auto prev_jt = data_parts_by_state_and_info.begin();
-        auto curr_jt = std::next(prev_jt);
-
     auto deactivate_part = [&] (DataPartIteratorByStateAndInfo it)
     {
+
         (*it)->remove_time.store((*it)->modification_time, std::memory_order_relaxed);
+        auto creation_csn = (*it)->version.creation_csn.load(std::memory_order_relaxed);
+        if (creation_csn != Tx::RolledBackCSN && creation_csn != Tx::PrehistoricCSN && !(*it)->version.isRemovalTIDLocked())
+        {
+            /// It's possible that covering part was created without transaction,
+            /// but if covered part was created with transaction (i.e. creation_tid is not prehistoric),
+            /// then it must have removal tid in metadata file.
+            throw Exception(ErrorCodes::LOGICAL_ERROR, "Data part {} is Outdated and has creation TID {} and CSN {}, "
+                            "but does not have removal tid. It's a bug or a result of manual intervention.",
+                            (*it)->name, (*it)->version.creation_tid, creation_csn);
+        }
         modifyPartState(it, DataPartState::Outdated);
         removePartContributionToDataVolume(*it);
     };
 
-        (*prev_jt)->assertState({DataPartState::Active});
+    /// All parts are in "Active" state after loading
+    assert(std::find_if(data_parts_by_state_and_info.begin(), data_parts_by_state_and_info.end(),
+    [](const auto & part)
+    {
+        return part->getState() != DataPartState::Active;
+    }) == data_parts_by_state_and_info.end());
 
-        while (curr_jt != data_parts_by_state_and_info.end() && (*curr_jt)->getState() == DataPartState::Active)
+    bool have_parts_with_version_metadata = false;
+    auto iter = data_parts_by_state_and_info.begin();
+    while (iter != data_parts_by_state_and_info.end() && (*iter)->getState() == DataPartState::Active)
     {
-            /// Don't consider data parts belonging to different partitions.
-            if ((*curr_jt)->info.partition_id != (*prev_jt)->info.partition_id)
+        const DataPartPtr & part = *iter;
+        part->loadVersionMetadata();
+        VersionMetadata & version = part->version;
+        if (part->wasInvolvedInTransaction())
         {
-                ++prev_jt;
-                ++curr_jt;
-                continue;
-            }
-
-            if ((*curr_jt)->contains(**prev_jt))
-            {
-                deactivate_part(prev_jt);
-                prev_jt = curr_jt;
-                ++curr_jt;
-            }
-            else if ((*prev_jt)->contains(**curr_jt))
-            {
-                auto next = std::next(curr_jt);
-                deactivate_part(curr_jt);
-                curr_jt = next;
+            have_parts_with_version_metadata = true;
         }
         else
         {
-                ++prev_jt;
-                ++curr_jt;
+            ++iter;
+            continue;
+        }
+
+        /// Check if CSNs were witten after committing transaction, update and write if needed.
+        bool version_updated = false;
+        assert(!version.creation_tid.isEmpty());
+        if (!part->version.creation_csn)
+        {
+            auto min = TransactionLog::getCSN(version.creation_tid);
+            if (!min)
+            {
+                /// Transaction that created this part was not committed. Remove part.
+                TransactionLog::assertTIDIsNotOutdated(version.creation_tid);
+                min = Tx::RolledBackCSN;
+            }
+            LOG_TRACE(log, "Will fix version metadata of {} after unclean restart: part has creation_tid={}, setting creation_csn={}",
+                      part->name, version.creation_tid, min);
+            version.creation_csn = min;
+            version_updated = true;
+        }
+        if (!version.removal_tid.isEmpty() && !part->version.removal_csn)
+        {
+            auto max = TransactionLog::getCSN(version.removal_tid);
+            if (max)
+            {
+                LOG_TRACE(log, "Will fix version metadata of {} after unclean restart: part has removal_tid={}, setting removal_csn={}",
+                          part->name, version.removal_tid, max);
+                version.removal_csn = max;
+            }
+            else
+            {
+                TransactionLog::assertTIDIsNotOutdated(version.removal_tid);
+                /// Transaction that tried to remove this part was not committed. Clear removal_tid.
+                LOG_TRACE(log, "Will fix version metadata of {} after unclean restart: clearing removal_tid={}",
+                          part->name, version.removal_tid);
+                version.unlockRemovalTID(version.removal_tid, TransactionInfoContext{getStorageID(), part->name});
+            }
+            version_updated = true;
+        }
+
+        /// Sanity checks
+        bool csn_order = !version.removal_csn || version.creation_csn <= version.removal_csn || version.removal_csn == Tx::PrehistoricCSN;
+        bool min_start_csn_order = version.creation_tid.start_csn <= version.creation_csn;
+        bool max_start_csn_order = version.removal_tid.start_csn <= version.removal_csn;
+        bool creation_csn_known = version.creation_csn;
+        if (!csn_order || !min_start_csn_order || !max_start_csn_order || !creation_csn_known)
+            throw Exception(ErrorCodes::LOGICAL_ERROR, "Part {} has invalid version metadata: {}", part->name, version.toString());
+
+        if (version_updated)
+            part->storeVersionMetadata();
+
+        /// Deactivate part if creation was not committed or if removal was.
+        if (version.creation_csn == Tx::RolledBackCSN || version.removal_csn)
+        {
+            auto next_it = std::next(iter);
+            deactivate_part(iter);
+            iter = next_it;
+        }
+        else
+        {
+            ++iter;
+        }
+    }
+
+    if (have_parts_with_version_metadata)
+        transactions_enabled.store(true);
+
+    /// Delete from the set of current parts those parts that are covered by another part (those parts that
+    /// were merged), but that for some reason are still not deleted from the filesystem.
+    /// Deletion of files will be performed later in the clearOldParts() method.
+
+    auto active_parts_range = getDataPartsStateRange(DataPartState::Active);
+    auto prev_it = active_parts_range.begin();
+    auto end_it = active_parts_range.end();
+
+    bool less_than_two_active_parts = prev_it == end_it || std::next(prev_it) == end_it;
+
+    if (!less_than_two_active_parts)
+    {
+        (*prev_it)->assertState({DataPartState::Active});
+        auto curr_it = std::next(prev_it);
+
+        while (curr_it != data_parts_by_state_and_info.end() && (*curr_it)->getState() == DataPartState::Active)
+        {
+            (*curr_it)->assertState({DataPartState::Active});
+
+            /// Don't consider data parts belonging to different partitions.
+            if ((*curr_it)->info.partition_id != (*prev_it)->info.partition_id)
+            {
+                ++prev_it;
+                ++curr_it;
+                continue;
+            }
+
+            if ((*curr_it)->contains(**prev_it))
+            {
+                deactivate_part(prev_it);
+                prev_it = curr_it;
+                ++curr_it;
+            }
+            else if ((*prev_it)->contains(**curr_it))
+            {
+                auto next = std::next(curr_it);
+                deactivate_part(curr_it);
+                curr_it = next;
+            }
+            else
+            {
+                ++prev_it;
+                ++curr_it;
             }
         }
     }
@@ -1482,12 +1586,20 @@ MergeTreeData::DataPartsVector MergeTreeData::grabOldParts(bool force)
         {
             const DataPartPtr & part = *it;
 
+            /// Do not remove outdated part if it may be visible for some transaction
+            if (!part->version.canBeRemoved())
+                continue;
+
             auto part_remove_time = part->remove_time.load(std::memory_order_relaxed);
 
-            if (part.unique() && /// Grab only parts that are not used by anyone (SELECTs for example).
-                ((part_remove_time < now &&
-                now - part_remove_time > getSettings()->old_parts_lifetime.totalSeconds()) || force
-                || isInMemoryPart(part))) /// Remove in-memory parts immediately to not store excessive data in RAM
+            /// Grab only parts that are not used by anyone (SELECTs for example).
+            if (!part.unique())
+                continue;
+
+            if ((part_remove_time < now && now - part_remove_time > getSettings()->old_parts_lifetime.totalSeconds())
+                || force
+                || isInMemoryPart(part)     /// Remove in-memory parts immediately to not store excessive data in RAM
+                || (part->version.creation_csn == Tx::RolledBackCSN && getSettings()->remove_rolled_back_parts_immediately))
             {
                 parts_to_delete.emplace_back(it);
             }
@@ -1578,7 +1690,7 @@ void MergeTreeData::flushAllInMemoryPartsIfNeeded()
         return;
 
     auto metadata_snapshot = getInMemoryMetadataPtr();
-    DataPartsVector parts = getDataPartsVector();
+    DataPartsVector parts = getDataPartsVectorForInternalUsage();
     for (const auto & part : parts)
     {
         if (auto part_in_memory = asInMemoryPart(part))
@@ -1641,7 +1753,7 @@ void MergeTreeData::clearPartsFromFilesystem(const DataPartsVector & parts_to_re
 
 size_t MergeTreeData::clearOldWriteAheadLogs()
 {
-    DataPartsVector parts = getDataPartsVector();
+    DataPartsVector parts = getDataPartsVectorForInternalUsage();
     std::vector<std::pair<Int64, Int64>> all_block_numbers_on_disk;
     std::vector<std::pair<Int64, Int64>> block_numbers_on_disk;
 
@@ -1706,15 +1818,20 @@ size_t MergeTreeData::clearEmptyParts()
         return 0;
 
     size_t cleared_count = 0;
-    auto parts = getDataPartsVector();
+    auto parts = getDataPartsVectorForInternalUsage();
     for (const auto & part : parts)
     {
-        if (part->rows_count == 0)
-        {
+        if (part->rows_count != 0)
+            continue;
+
+        /// Do not try to drop uncommitted parts.
+        if (!part->version.getCreationTID().isPrehistoric() && !part->version.isVisible(TransactionLog::instance().getLatestSnapshot()))
+            continue;
+
+        LOG_TRACE(log, "Will drop empty part {}", part->name);
         dropPartNoWaitNoThrow(part->name);
         ++cleared_count;
     }
-    }
     return cleared_count;
 }
 
@@ -2215,7 +2332,7 @@ void MergeTreeData::checkAlterIsPossible(const AlterCommands & commands, Context
         }
     }
 
-    for (const auto & part : getDataPartsVector())
+    for (const auto & part : getDataPartsVectorForInternalUsage())
     {
         bool at_least_one_column_rest = false;
         for (const auto & column : part->getColumns())
@@ -2500,6 +2617,7 @@ MergeTreeData::DataPartsVector MergeTreeData::getActivePartsToReplace(
 
 bool MergeTreeData::renameTempPartAndAdd(
     MutableDataPartPtr & part,
+    MergeTreeTransaction * txn,
     SimpleIncrement * increment,
     Transaction * out_transaction,
     MergeTreeDeduplicationLog * deduplication_log,
@@ -2512,7 +2630,7 @@ bool MergeTreeData::renameTempPartAndAdd(
     DataPartsVector covered_parts;
     {
         auto lock = lockParts();
-        if (!renameTempPartAndReplace(part, increment, out_transaction, lock, &covered_parts, deduplication_log, deduplication_token))
+        if (!renameTempPartAndReplace(part, txn, increment, out_transaction, lock, &covered_parts, deduplication_log, deduplication_token))
             return false;
     }
     if (!covered_parts.empty())
@@ -2525,6 +2643,7 @@ bool MergeTreeData::renameTempPartAndAdd(
 
 bool MergeTreeData::renameTempPartAndReplace(
     MutableDataPartPtr & part,
+    MergeTreeTransaction * txn,
     SimpleIncrement * increment,
     Transaction * out_transaction,
     std::unique_lock<std::mutex> & lock,
@@ -2536,6 +2655,9 @@ bool MergeTreeData::renameTempPartAndReplace(
         throw Exception("MergeTreeData::Transaction for one table cannot be used with another. It is a bug.",
             ErrorCodes::LOGICAL_ERROR);
 
+    if (txn)
+        transactions_enabled.store(true);
+
     part->assertState({DataPartState::Temporary});
 
     MergeTreePartInfo part_info = part->info;
@@ -2578,7 +2700,6 @@ bool MergeTreeData::renameTempPartAndReplace(
 
     DataPartPtr covering_part;
     DataPartsVector covered_parts = getActivePartsToReplace(part_info, part_name, covering_part, lock);
-    DataPartsVector covered_parts_in_memory;
 
     if (covering_part)
     {
@@ -2614,6 +2735,9 @@ bool MergeTreeData::renameTempPartAndReplace(
     part->renameTo(part_name, true);
 
     auto part_it = data_parts_indexes.insert(part).first;
+    /// FIXME Transactions: it's not the best place for checking and setting removal_tid,
+    /// because it's too optimistic. We should lock removal_tid of covered parts at the beginning of operation.
+    MergeTreeTransaction::addNewPartAndRemoveCovered(shared_from_this(), part, covered_parts, txn);
 
     if (out_transaction)
     {
@@ -2669,7 +2793,8 @@ bool MergeTreeData::renameTempPartAndReplace(
 }
 
 MergeTreeData::DataPartsVector MergeTreeData::renameTempPartAndReplace(
-    MutableDataPartPtr & part, SimpleIncrement * increment, Transaction * out_transaction, MergeTreeDeduplicationLog * deduplication_log)
+    MutableDataPartPtr & part, MergeTreeTransaction * txn, SimpleIncrement * increment,
+    Transaction * out_transaction, MergeTreeDeduplicationLog * deduplication_log)
 {
     if (out_transaction && &out_transaction->data != this)
         throw Exception("MergeTreeData::Transaction for one table cannot be used with another. It is a bug.",
@@ -2678,18 +2803,25 @@ MergeTreeData::DataPartsVector MergeTreeData::renameTempPartAndReplace(
     DataPartsVector covered_parts;
     {
         auto lock = lockParts();
-        renameTempPartAndReplace(part, increment, out_transaction, lock, &covered_parts, deduplication_log);
+        renameTempPartAndReplace(part, txn, increment, out_transaction, lock, &covered_parts, deduplication_log);
     }
     return covered_parts;
 }
 
-void MergeTreeData::removePartsFromWorkingSet(const MergeTreeData::DataPartsVector & remove, bool clear_without_timeout, DataPartsLock & acquired_lock)
+void MergeTreeData::removePartsFromWorkingSet(MergeTreeTransaction * txn, const MergeTreeData::DataPartsVector & remove, bool clear_without_timeout, DataPartsLock & acquired_lock)
+
 {
+    if (txn)
+        transactions_enabled.store(true);
+
     auto remove_time = clear_without_timeout ? 0 : time(nullptr);
     bool removed_active_part = false;
 
     for (const DataPartPtr & part : remove)
     {
+        if (part->version.creation_csn != Tx::RolledBackCSN)
+            MergeTreeTransaction::removeOldPart(shared_from_this(), part, txn);
+
         if (part->getState() == IMergeTreeDataPart::State::Active)
         {
             removePartContributionToColumnAndSecondaryIndexSizes(part);
@@ -2714,7 +2846,6 @@ void MergeTreeData::removePartsFromWorkingSet(const MergeTreeData::DataPartsVect
 void MergeTreeData::removePartsFromWorkingSetImmediatelyAndSetTemporaryState(const DataPartsVector & remove)
 {
     auto lock = lockParts();
-    bool removed_active_part = false;
 
     for (const auto & part : remove)
     {
@@ -2722,19 +2853,16 @@ void MergeTreeData::removePartsFromWorkingSetImmediatelyAndSetTemporaryState(con
         if (it_part == data_parts_by_info.end())
             throw Exception("Part " + part->getNameWithState() + " not found in data_parts", ErrorCodes::LOGICAL_ERROR);
 
-        if (part->getState() == IMergeTreeDataPart::State::Active)
-            removed_active_part = true;
+        assert(part->getState() == IMergeTreeDataPart::State::PreActive);
 
         modifyPartState(part, IMergeTreeDataPart::State::Temporary);
         /// Erase immediately
         data_parts_indexes.erase(it_part);
     }
-
-    if (removed_active_part)
-        resetObjectColumnsFromActiveParts(lock);
 }
 
-void MergeTreeData::removePartsFromWorkingSet(const DataPartsVector & remove, bool clear_without_timeout, DataPartsLock * acquired_lock)
+void MergeTreeData::removePartsFromWorkingSet(
+        MergeTreeTransaction * txn, const DataPartsVector & remove, bool clear_without_timeout, DataPartsLock * acquired_lock)
 {
     auto lock = (acquired_lock) ? DataPartsLock() : lockParts();
 
@@ -2746,11 +2874,12 @@ void MergeTreeData::removePartsFromWorkingSet(const DataPartsVector & remove, bo
         part->assertState({DataPartState::PreActive, DataPartState::Active, DataPartState::Outdated});
     }
 
-    removePartsFromWorkingSet(remove, clear_without_timeout, lock);
+    removePartsFromWorkingSet(txn, remove, clear_without_timeout, lock);
 }
 
-MergeTreeData::DataPartsVector MergeTreeData::removePartsInRangeFromWorkingSet(const MergeTreePartInfo & drop_range, bool clear_without_timeout,
-                                                                               DataPartsLock & lock)
+MergeTreeData::DataPartsVector MergeTreeData::removePartsInRangeFromWorkingSet(
+        MergeTreeTransaction * txn, const MergeTreePartInfo & drop_range,
+        bool clear_without_timeout, DataPartsLock & lock)
 {
     DataPartsVector parts_to_remove;
 
@@ -2815,15 +2944,34 @@ MergeTreeData::DataPartsVector MergeTreeData::removePartsInRangeFromWorkingSet(c
                             part->name, drop_range.getPartName());
         }
 
-        if (part->getState() != DataPartState::Deleting)
+        if (part->getState() == DataPartState::Deleting)
+            continue;
+
+        /// FIXME refactor removePartsFromWorkingSet(...), do not remove parts twice
+        if (txn)
+        {
+            if (!part->version.isVisible(*txn))
+                continue;
+        }
+
         parts_to_remove.emplace_back(part);
     }
 
-    removePartsFromWorkingSet(parts_to_remove, clear_without_timeout, lock);
+    removePartsFromWorkingSet(txn, parts_to_remove, clear_without_timeout, lock);
 
     return parts_to_remove;
 }
 
+void MergeTreeData::restoreAndActivatePart(const DataPartPtr & part, DataPartsLock * acquired_lock)
+{
+    auto lock = (acquired_lock) ? DataPartsLock() : lockParts();    //-V1018
+    if (part->getState() == DataPartState::Active)
+        return;
+    addPartContributionToColumnAndSecondaryIndexSizes(part);
+    addPartContributionToDataVolume(part);
+    modifyPartState(part, DataPartState::Active);
+}
+
 void MergeTreeData::forgetPartAndMoveToDetached(const MergeTreeData::DataPartPtr & part_to_detach, const String & prefix, bool restore_covered)
 {
     if (prefix.empty())
@@ -3238,9 +3386,23 @@ MergeTreeData::DataPartPtr MergeTreeData::getActiveContainingPart(const String &
     return getActiveContainingPart(part_info);
 }
 
-MergeTreeData::DataPartsVector MergeTreeData::getDataPartsVectorInPartition(MergeTreeData::DataPartState state, const String & partition_id) const
+MergeTreeData::DataPartsVector MergeTreeData::getVisibleDataPartsVectorInPartition(ContextPtr local_context, const String & partition_id) const
 {
-    DataPartStateAndPartitionID state_with_partition{state, partition_id};
+    if (const auto * txn = local_context->getCurrentTransaction().get())
+    {
+        DataPartStateAndPartitionID active_parts{MergeTreeDataPartState::Active, partition_id};
+        DataPartStateAndPartitionID outdated_parts{MergeTreeDataPartState::Outdated, partition_id};
+        DataPartsVector res;
+        {
+            auto lock = lockParts();
+            res.insert(res.end(), data_parts_by_state_and_info.lower_bound(active_parts), data_parts_by_state_and_info.upper_bound(active_parts));
+            res.insert(res.end(), data_parts_by_state_and_info.lower_bound(outdated_parts), data_parts_by_state_and_info.upper_bound(outdated_parts));
+        }
+        filterVisibleDataParts(res, txn->getSnapshot(), txn->tid);
+        return res;
+    }
+
+    DataPartStateAndPartitionID state_with_partition{MergeTreeDataPartState::Active, partition_id};
 
     auto lock = lockParts();
     return DataPartsVector(
@@ -3248,19 +3410,37 @@ MergeTreeData::DataPartsVector MergeTreeData::getDataPartsVectorInPartition(Merg
         data_parts_by_state_and_info.upper_bound(state_with_partition));
 }
 
-MergeTreeData::DataPartsVector MergeTreeData::getDataPartsVectorInPartitions(MergeTreeData::DataPartState state, const std::unordered_set<String> & partition_ids) const
+MergeTreeData::DataPartsVector MergeTreeData::getVisibleDataPartsVectorInPartitions(ContextPtr local_context, const std::unordered_set<String> & partition_ids) const
 {
-    auto lock = lockParts();
+    auto txn = local_context->getCurrentTransaction();
     DataPartsVector res;
+    {
+        auto lock = lockParts();
         for (const auto & partition_id : partition_ids)
         {
-        DataPartStateAndPartitionID state_with_partition{state, partition_id};
+            DataPartStateAndPartitionID active_parts{MergeTreeDataPartState::Active, partition_id};
             insertAtEnd(
                 res,
                 DataPartsVector(
-                data_parts_by_state_and_info.lower_bound(state_with_partition),
-                data_parts_by_state_and_info.upper_bound(state_with_partition)));
+                    data_parts_by_state_and_info.lower_bound(active_parts),
+                    data_parts_by_state_and_info.upper_bound(active_parts)));
+
+            if (txn)
+            {
+                DataPartStateAndPartitionID outdated_parts{MergeTreeDataPartState::Active, partition_id};
+
+                insertAtEnd(
+                    res,
+                    DataPartsVector(
+                        data_parts_by_state_and_info.lower_bound(outdated_parts),
+                        data_parts_by_state_and_info.upper_bound(outdated_parts)));
             }
+        }
+    }
+
+    if (txn)
+        filterVisibleDataParts(res, txn->getSnapshot(), txn->tid);
+
     return res;
 }
 
@@ -3292,6 +3472,8 @@ static void loadPartAndFixMetadataImpl(MergeTreeData::MutableDataPartPtr part)
 
     part->loadColumnsChecksumsIndexes(false, true);
     part->modification_time = disk->getLastModified(full_part_path).epochTime();
+    disk->removeFileIfExists(fs::path(full_part_path) / IMergeTreeDataPart::DELETE_ON_DESTROY_MARKER_FILE_NAME);
+    disk->removeFileIfExists(fs::path(full_part_path) / IMergeTreeDataPart::TXN_VERSION_METADATA_FILE_NAME);
 }
 
 void MergeTreeData::calculateColumnAndSecondaryIndexSizesImpl()
@@ -3398,16 +3580,16 @@ void MergeTreeData::checkAlterPartitionIsPossible(
     }
 }
 
-void MergeTreeData::checkPartitionCanBeDropped(const ASTPtr & partition)
+void MergeTreeData::checkPartitionCanBeDropped(const ASTPtr & partition, ContextPtr local_context)
 {
     DataPartsVector parts_to_remove;
     const auto * partition_ast = partition->as<ASTPartition>();
     if (partition_ast && partition_ast->all)
-        parts_to_remove = getDataPartsVector();
+        parts_to_remove = getVisibleDataPartsVector(local_context);
     else
     {
-        const String partition_id = getPartitionIDFromQuery(partition, getContext());
-        parts_to_remove = getDataPartsVectorInPartition(MergeTreeDataPartState::Active, partition_id);
+        const String partition_id = getPartitionIDFromQuery(partition, local_context);
+        parts_to_remove = getVisibleDataPartsVectorInPartition(local_context, partition_id);
     }
     UInt64 partition_size = 0;
 
@@ -3446,7 +3628,7 @@ void MergeTreeData::movePartitionToDisk(const ASTPtr & partition, const String &
             throw Exception("Part " + partition_id + " is not exists or not active", ErrorCodes::NO_SUCH_DATA_PART);
     }
     else
-        parts = getDataPartsVectorInPartition(MergeTreeDataPartState::Active, partition_id);
+        parts = getVisibleDataPartsVectorInPartition(local_context, partition_id);
 
     auto disk = getStoragePolicy()->getDiskByName(name);
     parts.erase(std::remove_if(parts.begin(), parts.end(), [&](auto part_ptr)
@@ -3488,7 +3670,7 @@ void MergeTreeData::movePartitionToVolume(const ASTPtr & partition, const String
             throw Exception("Part " + partition_id + " is not exists or not active", ErrorCodes::NO_SUCH_DATA_PART);
     }
     else
-        parts = getDataPartsVectorInPartition(MergeTreeDataPartState::Active, partition_id);
+        parts = getVisibleDataPartsVectorInPartition(local_context, partition_id);
 
     auto volume = getStoragePolicy()->getVolumeByName(name);
     if (!volume)
@@ -3560,7 +3742,7 @@ Pipe MergeTreeData::alterPartition(
                 }
                 else
                 {
-                    checkPartitionCanBeDropped(command.partition);
+                    checkPartitionCanBeDropped(command.partition, query_context);
                     dropPartition(command.partition, command.detach, query_context);
                 }
             }
@@ -3609,7 +3791,7 @@ Pipe MergeTreeData::alterPartition(
             case PartitionCommand::REPLACE_PARTITION:
             {
                 if (command.replace)
-                    checkPartitionCanBeDropped(command.partition);
+                    checkPartitionCanBeDropped(command.partition, query_context);
                 String from_database = query_context->resolveDatabase(command.from_database);
                 auto from_storage = DatabaseCatalog::instance().getTable({from_database, command.from_table}, query_context);
                 replacePartitionFrom(from_storage, command.partition, command.replace, query_context);
@@ -3668,9 +3850,9 @@ BackupEntries MergeTreeData::backupData(ContextPtr local_context, const ASTs & p
 {
     DataPartsVector data_parts;
     if (partitions.empty())
-        data_parts = getDataPartsVector();
+        data_parts = getVisibleDataPartsVector(local_context);
     else
-        data_parts = getDataPartsVectorInPartitions(MergeTreeDataPartState::Active, getPartitionIDsFromQuery(partitions, local_context));
+        data_parts = getVisibleDataPartsVectorInPartitions(local_context, getPartitionIDsFromQuery(partitions, local_context));
     return backupDataParts(data_parts);
 }
 
@@ -3807,8 +3989,11 @@ private:
 
             auto single_disk_volume = std::make_shared<SingleDiskVolume>(disk->getName(), disk, 0);
             auto part = storage->createPart(part_name, part_info, single_disk_volume, relative_temp_part_dir);
+            /// TODO Transactions: Decide what to do with version metadata (if any). Let's just remove it for now.
+            disk->removeFileIfExists(fs::path(temp_part_dir) / IMergeTreeDataPart::TXN_VERSION_METADATA_FILE_NAME);
+            part->version.setCreationTID(Tx::PrehistoricTID, nullptr);
             part->loadColumnsChecksumsIndexes(false, true);
-            storage->renameTempPartAndAdd(part, increment);
+            storage->renameTempPartAndAdd(part, NO_TRANSACTION_RAW, increment);
             return {};
         }
 
@@ -3928,6 +4113,86 @@ String MergeTreeData::getPartitionIDFromQuery(const ASTPtr & ast, ContextPtr loc
     return partition_id;
 }
 
+
+DataPartsVector MergeTreeData::getVisibleDataPartsVector(ContextPtr local_context) const
+{
+    DataPartsVector res;
+    if (const auto * txn = local_context->getCurrentTransaction().get())
+    {
+        res = getDataPartsVectorForInternalUsage({DataPartState::Active, DataPartState::Outdated});
+        filterVisibleDataParts(res, txn->getSnapshot(), txn->tid);
+    }
+    else
+    {
+        res = getDataPartsVectorForInternalUsage();
+    }
+    return res;
+}
+
+DataPartsVector MergeTreeData::getVisibleDataPartsVectorUnlocked(ContextPtr local_context, const DataPartsLock & lock) const
+{
+    DataPartsVector res;
+    if (const auto * txn = local_context->getCurrentTransaction().get())
+    {
+        res = getDataPartsVectorForInternalUsage({DataPartState::Active, DataPartState::Outdated}, lock);
+        filterVisibleDataParts(res, txn->getSnapshot(), txn->tid);
+    }
+    else
+    {
+        res = getDataPartsVectorForInternalUsage({DataPartState::Active}, lock);
+    }
+    return res;
+}
+
+MergeTreeData::DataPartsVector MergeTreeData::getVisibleDataPartsVector(const MergeTreeTransactionPtr & txn) const
+{
+    DataPartsVector res;
+    if (txn)
+    {
+        res = getDataPartsVectorForInternalUsage({DataPartState::Active, DataPartState::Outdated});
+        filterVisibleDataParts(res, txn->getSnapshot(), txn->tid);
+    }
+    else
+    {
+        res = getDataPartsVectorForInternalUsage();
+    }
+    return res;
+}
+
+MergeTreeData::DataPartsVector MergeTreeData::getVisibleDataPartsVector(CSN snapshot_version, TransactionID current_tid) const
+{
+    auto res = getDataPartsVectorForInternalUsage({DataPartState::Active, DataPartState::Outdated});
+    filterVisibleDataParts(res, snapshot_version, current_tid);
+    return res;
+}
+
+void MergeTreeData::filterVisibleDataParts(DataPartsVector & maybe_visible_parts, CSN snapshot_version, TransactionID current_tid) const
+{
+    [[maybe_unused]] size_t total_size = maybe_visible_parts.size();
+
+    auto need_remove_pred = [snapshot_version, &current_tid] (const DataPartPtr & part) -> bool
+    {
+        return !part->version.isVisible(snapshot_version, current_tid);
+    };
+
+    auto new_end_it = std::remove_if(maybe_visible_parts.begin(), maybe_visible_parts.end(), need_remove_pred);
+    maybe_visible_parts.erase(new_end_it, maybe_visible_parts.end());
+    [[maybe_unused]] size_t visible_size = maybe_visible_parts.size();
+
+
+    auto get_part_names = [&maybe_visible_parts]() -> Strings
+    {
+        Strings visible_part_names;
+        for (const auto & p : maybe_visible_parts)
+            visible_part_names.push_back(p->name);
+        return visible_part_names;
+    };
+
+    LOG_TEST(log, "Got {} parts (of {}) visible in snapshot {} (TID {}): {}",
+             visible_size, total_size, snapshot_version, current_tid, fmt::join(get_part_names(), ", "));
+}
+
+
 std::unordered_set<String> MergeTreeData::getPartitionIDsFromQuery(const ASTs & asts, ContextPtr local_context) const
 {
     std::unordered_set<String> partition_ids;
@@ -3958,7 +4223,7 @@ std::set<String> MergeTreeData::getPartitionIdsAffectedByCommands(
 }
 
 
-MergeTreeData::DataPartsVector MergeTreeData::getDataPartsVectorUnlocked(
+MergeTreeData::DataPartsVector MergeTreeData::getDataPartsVectorForInternalUsage(
     const DataPartStates & affordable_states,
     const DataPartsLock & /*lock*/,
     DataPartStateVector * out_states,
@@ -4005,13 +4270,13 @@ MergeTreeData::DataPartsVector MergeTreeData::getDataPartsVectorUnlocked(
     return res;
 }
 
-MergeTreeData::DataPartsVector MergeTreeData::getDataPartsVector(
+MergeTreeData::DataPartsVector MergeTreeData::getDataPartsVectorForInternalUsage(
     const DataPartStates & affordable_states,
     DataPartStateVector * out_states,
     bool require_projection_parts) const
 {
     auto lock = lockParts();
-    return getDataPartsVectorUnlocked(affordable_states, lock, out_states, require_projection_parts);
+    return getDataPartsVectorForInternalUsage(affordable_states, lock, out_states, require_projection_parts);
 }
 
 MergeTreeData::DataPartsVector
@@ -4371,14 +4636,14 @@ MergeTreeData::DataParts MergeTreeData::getDataParts(const DataPartStates & affo
     return res;
 }
 
-MergeTreeData::DataParts MergeTreeData::getDataParts() const
+MergeTreeData::DataParts MergeTreeData::getDataPartsForInternalUsage() const
 {
     return getDataParts({DataPartState::Active});
 }
 
-MergeTreeData::DataPartsVector MergeTreeData::getDataPartsVector() const
+MergeTreeData::DataPartsVector MergeTreeData::getDataPartsVectorForInternalUsage() const
 {
-    return getDataPartsVector({DataPartState::Active});
+    return getDataPartsVectorForInternalUsage({DataPartState::Active});
 }
 
 MergeTreeData::DataPartPtr MergeTreeData::getAnyPartInPartition(
@@ -4422,7 +4687,19 @@ void MergeTreeData::Transaction::rollback()
         buf << ".";
         LOG_DEBUG(data.log, "Undoing transaction.{}", buf.str());
 
-        data.removePartsFromWorkingSet(
+        if (!txn)
+        {
+            auto lock = data.lockParts();
+            for (const auto & part : precommitted_parts)
+            {
+                DataPartPtr covering_part;
+                DataPartsVector covered_parts = data.getActivePartsToReplace(part->info, part->name, covering_part, lock);
+                for (auto & covered : covered_parts)
+                    covered->version.unlockRemovalTID(Tx::PrehistoricTID, TransactionInfoContext{data.getStorageID(), covered->name});
+            }
+        }
+
+        data.removePartsFromWorkingSet(txn,
             DataPartsVector(precommitted_parts.begin(), precommitted_parts.end()),
             /* clear_without_timeout = */ true);
     }
@@ -5413,7 +5690,8 @@ MergeTreeData::MutableDataPartPtr MergeTreeData::cloneAndLoadDataPartOnSameDisk(
     const MergeTreeData::DataPartPtr & src_part,
     const String & tmp_part_prefix,
     const MergeTreePartInfo & dst_part_info,
-    const StorageMetadataPtr & metadata_snapshot)
+    const StorageMetadataPtr & metadata_snapshot,
+    const MergeTreeTransactionPtr & txn)
 {
     /// Check that the storage policy contains the disk where the src_part is located.
     bool does_storage_policy_allow_same_disk = false;
@@ -5431,6 +5709,7 @@ MergeTreeData::MutableDataPartPtr MergeTreeData::cloneAndLoadDataPartOnSameDisk(
             ErrorCodes::BAD_ARGUMENTS);
 
     String dst_part_name = src_part->getNewName(dst_part_info);
+    assert(!tmp_part_prefix.empty());
     String tmp_dst_part_name = tmp_part_prefix + dst_part_name;
 
     auto reservation = reserveSpace(src_part->getBytesOnDisk(), src_part->volume->getDisk());
@@ -5451,12 +5730,18 @@ MergeTreeData::MutableDataPartPtr MergeTreeData::cloneAndLoadDataPartOnSameDisk(
     }
 
     LOG_DEBUG(log, "Cloning part {} to {}", fullPath(disk, src_part_path), fullPath(disk, dst_part_path));
-    localBackup(disk, src_part_path, dst_part_path);
+    localBackup(disk, src_part_path, dst_part_path, /* make_source_readonly */ false);
     disk->removeFileIfExists(fs::path(dst_part_path) / IMergeTreeDataPart::DELETE_ON_DESTROY_MARKER_FILE_NAME);
+    disk->removeFileIfExists(fs::path(dst_part_path) / IMergeTreeDataPart::TXN_VERSION_METADATA_FILE_NAME);
 
     auto single_disk_volume = std::make_shared<SingleDiskVolume>(disk->getName(), disk, 0);
     auto dst_data_part = createPart(dst_part_name, dst_part_info, single_disk_volume, tmp_dst_part_name);
 
+    /// We should write version metadata on part creation to distinguish it from parts that were created without transaction.
+    TransactionID tid = txn ? txn->tid : Tx::PrehistoricTID;
+    dst_data_part->version.setCreationTID(tid, nullptr);
+    dst_data_part->storeVersionMetadata();
+
     dst_data_part->is_temp = true;
 
     dst_data_part->loadColumnsChecksumsIndexes(require_part_metadata, true);
@@ -5505,7 +5790,7 @@ void MergeTreeData::reportBrokenPart(MergeTreeData::DataPartPtr & data_part) con
     if (data_part->volume && data_part->volume->getDisk()->isBroken())
     {
         auto disk = data_part->volume->getDisk();
-        auto parts = getDataParts();
+        auto parts = getDataPartsForInternalUsage();
         LOG_WARNING(log, "Scanning parts to recover on broken disk {}.", disk->getName() + "@" + disk->getPath());
         for (const auto & part : parts)
         {
@@ -5580,7 +5865,7 @@ PartitionCommandsResultInfo MergeTreeData::freezePartitionsByMatcher(
     const String shadow_path = "shadow/";
 
     /// Acquire a snapshot of active data parts to prevent removing while doing backup.
-    const auto data_parts = getDataParts();
+    const auto data_parts = getVisibleDataPartsVector(local_context);
 
     String backup_name = (!with_name.empty() ? escapeForFileName(with_name) : toString(increment));
     String backup_path = fs::path(shadow_path) / backup_name / "";
@@ -6323,12 +6608,12 @@ void MergeTreeData::updateObjectColumns(const DataPartPtr & part, const DataPart
     DB::updateObjectColumns(object_columns, part->getColumns());
 }
 
-StorageSnapshotPtr MergeTreeData::getStorageSnapshot(const StorageMetadataPtr & metadata_snapshot) const
+StorageSnapshotPtr MergeTreeData::getStorageSnapshot(const StorageMetadataPtr & metadata_snapshot, ContextPtr query_context) const
 {
     auto snapshot_data = std::make_unique<SnapshotData>();
 
     auto lock = lockParts();
-    snapshot_data->parts = getDataPartsVectorUnlocked({DataPartState::Active}, lock);
+    snapshot_data->parts = getVisibleDataPartsVectorUnlocked(query_context, lock);
     return std::make_shared<StorageSnapshot>(*this, metadata_snapshot, object_columns, std::move(snapshot_data));
 }
 

src/Storages/MergeTree/MergeTreeData.h

@@ -49,6 +49,7 @@ class MutationCommands;
 class Context;
 using PartitionIdToMaxBlock = std::unordered_map<String, Int64>;
 struct JobAndPool;
+class MergeTreeTransaction;
 struct ZeroCopyLock;
 
 /// Auxiliary struct holding information about the future merged or mutated part.
@@ -247,7 +248,7 @@ public:
     class Transaction : private boost::noncopyable
     {
     public:
-        explicit Transaction(MergeTreeData & data_) : data(data_) {}
+        Transaction(MergeTreeData & data_, MergeTreeTransaction * txn_) : data(data_), txn(txn_) {}
 
         DataPartsVector commit(MergeTreeData::DataPartsLock * acquired_parts_lock = nullptr);
 
@@ -276,6 +277,7 @@ public:
         friend class MergeTreeData;
 
         MergeTreeData & data;
+        MergeTreeTransaction * txn;
         DataParts precommitted_parts;
 
         void clear() { precommitted_parts.clear(); }
@@ -436,7 +438,7 @@ public:
         DataPartsVector parts;
     };
 
-    StorageSnapshotPtr getStorageSnapshot(const StorageMetadataPtr & metadata_snapshot) const override;
+    StorageSnapshotPtr getStorageSnapshot(const StorageMetadataPtr & metadata_snapshot, ContextPtr query_context) const override;
 
     /// Load the set of data parts from disk. Call once - immediately after the object is created.
     void loadDataParts(bool skip_sanity_checks);
@@ -445,10 +447,11 @@ public:
 
     Int64 getMaxBlockNumber() const;
 
+
     /// Returns a copy of the list so that the caller shouldn't worry about locks.
     DataParts getDataParts(const DataPartStates & affordable_states) const;
 
-    DataPartsVector getDataPartsVectorUnlocked(
+    DataPartsVector getDataPartsVectorForInternalUsage(
         const DataPartStates & affordable_states,
         const DataPartsLock & lock,
         DataPartStateVector * out_states = nullptr,
@@ -456,31 +459,28 @@ public:
 
     /// Returns sorted list of the parts with specified states
     ///  out_states will contain snapshot of each part state
-    DataPartsVector getDataPartsVector(
-        const DataPartStates & affordable_states,
-        DataPartStateVector * out_states = nullptr,
-        bool require_projection_parts = false) const;
+    DataPartsVector getDataPartsVectorForInternalUsage(
+        const DataPartStates & affordable_states, DataPartStateVector * out_states = nullptr, bool require_projection_parts = false) const;
+
 
     /// Returns absolutely all parts (and snapshot of their states)
     DataPartsVector getAllDataPartsVector(
         DataPartStateVector * out_states = nullptr,
         bool require_projection_parts = false) const;
 
-    /// Returns all detached parts
-    DetachedPartsInfo getDetachedParts() const;
+    /// Returns parts in Active state
+    DataParts getDataPartsForInternalUsage() const;
+    DataPartsVector getDataPartsVectorForInternalUsage() const;
 
-    static void validateDetachedPartName(const String & name);
+    void filterVisibleDataParts(DataPartsVector & maybe_visible_parts, CSN snapshot_version, TransactionID current_tid) const;
 
-    void dropDetached(const ASTPtr & partition, bool part, ContextPtr local_context);
+    /// Returns parts that visible with current snapshot
+    DataPartsVector getVisibleDataPartsVector(ContextPtr local_context) const;
+    DataPartsVector getVisibleDataPartsVectorUnlocked(ContextPtr local_context, const DataPartsLock & lock) const;
+    DataPartsVector getVisibleDataPartsVector(const MergeTreeTransactionPtr & txn) const;
+    DataPartsVector getVisibleDataPartsVector(CSN snapshot_version, TransactionID current_tid) const;
 
-    MutableDataPartsVector tryLoadPartsToAttach(const ASTPtr & partition, bool attach_part,
-            ContextPtr context, PartsTemporaryRename & renamed_parts);
-
-    /// Returns Active parts
-    DataParts getDataParts() const;
-    DataPartsVector getDataPartsVector() const;
-
-    /// Returns a committed part with the given name or a part containing it. If there is no such part, returns nullptr.
+    /// Returns a part in Active state with the given name or a part containing it. If there is no such part, returns nullptr.
     DataPartPtr getActiveContainingPart(const String & part_name) const;
     DataPartPtr getActiveContainingPart(const MergeTreePartInfo & part_info) const;
     DataPartPtr getActiveContainingPart(const MergeTreePartInfo & part_info, DataPartState state, DataPartsLock & lock) const;
@@ -490,8 +490,8 @@ public:
     void swapActivePart(MergeTreeData::DataPartPtr part_copy);
 
     /// Returns all parts in specified partition
-    DataPartsVector getDataPartsVectorInPartition(DataPartState state, const String & partition_id) const;
-    DataPartsVector getDataPartsVectorInPartitions(DataPartState state, const std::unordered_set<String> & partition_ids) const;
+    DataPartsVector getVisibleDataPartsVectorInPartition(ContextPtr local_context, const String & partition_id) const;
+    DataPartsVector getVisibleDataPartsVectorInPartitions(ContextPtr local_context, const std::unordered_set<String> & partition_ids) const;
 
     /// Returns the part with the given name and state or nullptr if no such part.
     DataPartPtr getPartIfExists(const String & part_name, const DataPartStates & valid_states);
@@ -511,6 +511,18 @@ public:
     /// Makes sense only for ordinary MergeTree engines because for them block numbering doesn't depend on partition.
     std::optional<Int64> getMinPartDataVersion() const;
 
+
+    /// Returns all detached parts
+    DetachedPartsInfo getDetachedParts() const;
+
+    static void validateDetachedPartName(const String & name);
+
+    void dropDetached(const ASTPtr & partition, bool part, ContextPtr context);
+
+    MutableDataPartsVector tryLoadPartsToAttach(const ASTPtr & partition, bool attach_part,
+                                                ContextPtr context, PartsTemporaryRename & renamed_parts);
+
+
     /// If the table contains too many active parts, sleep for a while to give them time to merge.
     /// If until is non-null, wake up from the sleep earlier if the event happened.
     void delayInsertOrThrowIfNeeded(Poco::Event * until = nullptr) const;
@@ -524,6 +536,7 @@ public:
     /// Returns true if part was added. Returns false if part is covered by bigger part.
     bool renameTempPartAndAdd(
         MutableDataPartPtr & part,
+        MergeTreeTransaction * txn,
         SimpleIncrement * increment = nullptr,
         Transaction * out_transaction = nullptr,
         MergeTreeDeduplicationLog * deduplication_log = nullptr,
@@ -533,11 +546,14 @@ public:
     /// Returns all parts covered by the added part (in ascending order).
     /// If out_transaction == nullptr, marks covered parts as Outdated.
     DataPartsVector renameTempPartAndReplace(
-        MutableDataPartPtr & part, SimpleIncrement * increment = nullptr, Transaction * out_transaction = nullptr, MergeTreeDeduplicationLog * deduplication_log = nullptr);
+        MutableDataPartPtr & part, MergeTreeTransaction * txn, SimpleIncrement * increment = nullptr,
+        Transaction * out_transaction = nullptr, MergeTreeDeduplicationLog * deduplication_log = nullptr);
 
     /// Low-level version of previous one, doesn't lock mutex
+    /// FIXME Transactions: remove add_to_txn flag, maybe merge MergeTreeTransaction and Transaction
     bool renameTempPartAndReplace(
         MutableDataPartPtr & part,
+        MergeTreeTransaction * txn,
         SimpleIncrement * increment,
         Transaction * out_transaction,
         DataPartsLock & lock,
@@ -554,15 +570,18 @@ public:
     /// Parts in add must already be in data_parts with PreActive, Active, or Outdated states.
     /// If clear_without_timeout is true, the parts will be deleted at once, or during the next call to
     /// clearOldParts (ignoring old_parts_lifetime).
-    void removePartsFromWorkingSet(const DataPartsVector & remove, bool clear_without_timeout, DataPartsLock * acquired_lock = nullptr);
-    void removePartsFromWorkingSet(const DataPartsVector & remove, bool clear_without_timeout, DataPartsLock & acquired_lock);
+    void removePartsFromWorkingSet(MergeTreeTransaction * txn, const DataPartsVector & remove, bool clear_without_timeout, DataPartsLock * acquired_lock = nullptr);
+    void removePartsFromWorkingSet(MergeTreeTransaction * txn, const DataPartsVector & remove, bool clear_without_timeout, DataPartsLock & acquired_lock);
 
     /// Removes all parts from the working set parts
     ///  for which (partition_id = drop_range.partition_id && min_block >= drop_range.min_block && max_block <= drop_range.max_block).
     /// Used in REPLACE PARTITION command;
-    DataPartsVector removePartsInRangeFromWorkingSet(const MergeTreePartInfo & drop_range, bool clear_without_timeout,
+    DataPartsVector removePartsInRangeFromWorkingSet(MergeTreeTransaction * txn, const MergeTreePartInfo & drop_range, bool clear_without_timeout,
                                                      DataPartsLock & lock);
 
+    /// Restores Outdated part and adds it to working set
+    void restoreAndActivatePart(const DataPartPtr & part, DataPartsLock * acquired_lock = nullptr);
+
     /// Renames the part to detached/<prefix>_<part> and removes it from data_parts,
     //// so it will not be deleted in clearOldParts.
     /// If restore_covered is true, adds to the working set inactive parts, which were merged into the deleted part.
@@ -697,7 +716,10 @@ public:
     /// Moves partition to specified Volume
     void movePartitionToVolume(const ASTPtr & partition, const String & name, bool moving_part, ContextPtr context);
 
-    void checkPartitionCanBeDropped(const ASTPtr & partition) override;
+    /// Checks that Partition could be dropped right now
+    /// Otherwise - throws an exception with detailed information.
+    /// We do not use mutex because it is not very important that the size could change during the operation.
+    void checkPartitionCanBeDropped(const ASTPtr & partition, ContextPtr local_context);
 
     void checkPartCanBeDropped(const String & part_name);
 
@@ -743,7 +765,8 @@ public:
     MergeTreeData & checkStructureAndGetMergeTreeData(IStorage & source_table, const StorageMetadataPtr & src_snapshot, const StorageMetadataPtr & my_snapshot) const;
 
     MergeTreeData::MutableDataPartPtr cloneAndLoadDataPartOnSameDisk(
-        const MergeTreeData::DataPartPtr & src_part, const String & tmp_part_prefix, const MergeTreePartInfo & dst_part_info, const StorageMetadataPtr & metadata_snapshot);
+        const MergeTreeData::DataPartPtr & src_part, const String & tmp_part_prefix, const MergeTreePartInfo & dst_part_info,
+        const StorageMetadataPtr & metadata_snapshot, const MergeTreeTransactionPtr & txn);
 
     virtual std::vector<MergeTreeMutationStatus> getMutationsStatus() const = 0;
 
@@ -978,6 +1001,9 @@ protected:
     mutable std::shared_mutex pinned_part_uuids_mutex;
     PinnedPartUUIDsPtr pinned_part_uuids;
 
+    /// True if at least one part was created/removed with transaction.
+    mutable std::atomic_bool transactions_enabled = false;
+
     /// Work with data parts
 
     struct TagByInfo{};

src/Storages/MergeTree/MergeTreeDataMergerMutator.cpp

@@ -14,6 +14,7 @@
 #include <Storages/MergeTree/MergeTreeData.h>
 #include <Storages/MergeTree/MergeProgress.h>
 #include <Storages/MergeTree/MergeTask.h>
+#include <Storages/MergeTree/ActiveDataPartSet.h>
 
 #include <Processors/Transforms/TTLTransform.h>
 #include <Processors/Transforms/TTLCalcTransform.h>
@@ -29,6 +30,7 @@
 #include <Processors/Transforms/ExpressionTransform.h>
 #include <Processors/Transforms/MaterializingTransform.h>
 #include <Interpreters/MutationsInterpreter.h>
+#include <Interpreters/MergeTreeTransaction.h>
 #include <Interpreters/Context.h>
 #include <Common/interpolate.h>
 #include <Common/typeid_cast.h>
@@ -52,6 +54,7 @@ namespace DB
 namespace ErrorCodes
 {
     extern const int LOGICAL_ERROR;
+    extern const int ABORTED;
 }
 
 /// Do not start to merge parts, if free space is less than sum size of parts times specified coefficient.
@@ -124,9 +127,70 @@ SelectPartsDecision MergeTreeDataMergerMutator::selectPartsToMerge(
     size_t max_total_size_to_merge,
     const AllowedMergingPredicate & can_merge_callback,
     bool merge_with_ttl_allowed,
+    const MergeTreeTransactionPtr & txn,
     String * out_disable_reason)
 {
-    MergeTreeData::DataPartsVector data_parts = data.getDataPartsVector();
+    MergeTreeData::DataPartsVector data_parts;
+    if (txn)
+    {
+        /// Merge predicate (for simple MergeTree) allows to merge two parts only if both parts are visible for merge transaction.
+        /// So at the first glance we could just get all active parts.
+        /// Active parts include uncommitted parts, but it's ok and merge predicate handles it.
+        /// However, it's possible that some transaction is trying to remove a part in the middle, for example, all_2_2_0.
+        /// If parts all_1_1_0 and all_3_3_0 are active and visible for merge transaction, then we would try to merge them.
+        /// But it's wrong, because all_2_2_0 may become active again if transaction will roll back.
+        /// That's why we must include some outdated parts into `data_part`, more precisely, such parts that removal is not committed.
+        MergeTreeData::DataPartsVector active_parts;
+        MergeTreeData::DataPartsVector outdated_parts;
+
+        {
+            auto lock = data.lockParts();
+            active_parts = data.getDataPartsVectorForInternalUsage({MergeTreeData::DataPartState::Active}, lock);
+            outdated_parts = data.getDataPartsVectorForInternalUsage({MergeTreeData::DataPartState::Outdated}, lock);
+        }
+
+        ActiveDataPartSet active_parts_set{data.format_version};
+        for (const auto & part : active_parts)
+            active_parts_set.add(part->name);
+
+        for (const auto & part : outdated_parts)
+        {
+            /// We don't need rolled back parts.
+            /// NOTE When rolling back a transaction we set creation_csn to RolledBackCSN at first
+            /// and then remove part from working set, so there's no race condition
+            if (part->version.creation_csn == Tx::RolledBackCSN)
+                continue;
+
+            /// We don't need parts that are finally removed.
+            /// NOTE There's a minor race condition: we may get UnknownCSN if a transaction has been just committed concurrently.
+            /// But it's not a problem if we will add such part to `data_parts`.
+            if (part->version.removal_csn != Tx::UnknownCSN)
+                continue;
+
+            active_parts_set.add(part->name);
+        }
+
+        /// Restore "active" parts set from selected active and outdated parts
+        auto remove_pred = [&](const MergeTreeData::DataPartPtr & part) -> bool
+        {
+            return active_parts_set.getContainingPart(part->info) != part->name;
+        };
+
+        auto new_end_it = std::remove_if(active_parts.begin(), active_parts.end(), remove_pred);
+        active_parts.erase(new_end_it, active_parts.end());
+
+        new_end_it = std::remove_if(outdated_parts.begin(), outdated_parts.end(), remove_pred);
+        outdated_parts.erase(new_end_it, outdated_parts.end());
+
+        std::merge(active_parts.begin(), active_parts.end(),
+                   outdated_parts.begin(), outdated_parts.end(),
+                   std::back_inserter(data_parts), MergeTreeData::LessDataPart());
+    }
+    else
+    {
+        /// Simply get all active parts
+        data_parts = data.getDataPartsVectorForInternalUsage();
+    }
     const auto data_settings = data.getSettings();
     auto metadata_snapshot = data.getInMemoryMetadataPtr();
 
@@ -172,7 +236,7 @@ SelectPartsDecision MergeTreeDataMergerMutator::selectPartsToMerge(
             * So we have to check if this part is currently being inserted with quorum and so on and so forth.
             * Obviously we have to check it manually only for the first part
             * of each partition because it will be automatically checked for a pair of parts. */
-            if (!can_merge_callback(nullptr, part, nullptr))
+            if (!can_merge_callback(nullptr, part, txn.get(), nullptr))
                 continue;
 
             /// This part can be merged only with next parts (no prev part exists), so start
@@ -184,7 +248,7 @@ SelectPartsDecision MergeTreeDataMergerMutator::selectPartsToMerge(
         {
             /// If we cannot merge with previous part we had to start new parts
             /// interval (in the same partition)
-            if (!can_merge_callback(*prev_part, part, nullptr))
+            if (!can_merge_callback(*prev_part, part, txn.get(), nullptr))
             {
                 /// Now we have no previous part
                 prev_part = nullptr;
@@ -196,7 +260,7 @@ SelectPartsDecision MergeTreeDataMergerMutator::selectPartsToMerge(
                 /// for example, merge is already assigned for such parts, or they participate in quorum inserts
                 /// and so on.
                 /// Also we don't start new interval here (maybe all next parts cannot be merged and we don't want to have empty interval)
-                if (!can_merge_callback(nullptr, part, nullptr))
+                if (!can_merge_callback(nullptr, part, txn.get(), nullptr))
                     continue;
 
                 /// Starting new interval in the same partition
@@ -307,6 +371,7 @@ SelectPartsDecision MergeTreeDataMergerMutator::selectAllPartsToMergeWithinParti
     const String & partition_id,
     bool final,
     const StorageMetadataPtr & metadata_snapshot,
+    const MergeTreeTransactionPtr & txn,
     String * out_disable_reason,
     bool optimize_skip_merged_partitions)
 {
@@ -343,7 +408,7 @@ SelectPartsDecision MergeTreeDataMergerMutator::selectAllPartsToMergeWithinParti
     while (it != parts.end())
     {
         /// For the case of one part, we check that it can be merged "with itself".
-        if ((it != parts.begin() || parts.size() == 1) && !can_merge(*prev_it, *it, out_disable_reason))
+        if ((it != parts.begin() || parts.size() == 1) && !can_merge(*prev_it, *it, txn.get(), out_disable_reason))
         {
             return SelectPartsDecision::CANNOT_SELECT;
         }
@@ -390,7 +455,7 @@ MergeTreeData::DataPartsVector MergeTreeDataMergerMutator::selectAllPartsFromPar
 {
     MergeTreeData::DataPartsVector parts_from_partition;
 
-    MergeTreeData::DataParts data_parts = data.getDataParts();
+    MergeTreeData::DataParts data_parts = data.getDataPartsForInternalUsage();
 
     for (const auto & current_part : data_parts)
     {
@@ -416,6 +481,7 @@ MergeTaskPtr MergeTreeDataMergerMutator::mergePartsToTemporaryPart(
     bool deduplicate,
     const Names & deduplicate_by_columns,
     const MergeTreeData::MergingParams & merging_params,
+    const MergeTreeTransactionPtr & txn,
     const IMergeTreeDataPart * parent_part,
     const String & suffix)
 {
@@ -432,6 +498,7 @@ MergeTaskPtr MergeTreeDataMergerMutator::mergePartsToTemporaryPart(
         merging_params,
         parent_part,
         suffix,
+        txn,
         &data,
         this,
         &merges_blocker,
@@ -446,6 +513,7 @@ MutateTaskPtr MergeTreeDataMergerMutator::mutatePartToTemporaryPart(
     MergeListEntry * merge_entry,
     time_t time_of_mutation,
     ContextPtr context,
+    const MergeTreeTransactionPtr & txn,
     ReservationSharedPtr space_reservation,
     TableLockHolder & holder)
 {
@@ -458,6 +526,7 @@ MutateTaskPtr MergeTreeDataMergerMutator::mutatePartToTemporaryPart(
         context,
         space_reservation,
         holder,
+        txn,
         data,
         *this,
         merges_blocker
@@ -508,10 +577,16 @@ MergeAlgorithm MergeTreeDataMergerMutator::chooseMergeAlgorithm(
 MergeTreeData::DataPartPtr MergeTreeDataMergerMutator::renameMergedTemporaryPart(
     MergeTreeData::MutableDataPartPtr & new_data_part,
     const MergeTreeData::DataPartsVector & parts,
+    const MergeTreeTransactionPtr & txn,
     MergeTreeData::Transaction * out_transaction)
 {
+    /// Some of source parts was possibly created in transaction, so non-transactional merge may break isolation.
+    if (data.transactions_enabled.load(std::memory_order_relaxed) && !txn)
+        throw Exception(ErrorCodes::ABORTED, "Cancelling merge, because it was done without starting transaction,"
+                                             "but transactions were enabled for this table");
+
     /// Rename new part, add to the set and remove original parts.
-    auto replaced_parts = data.renameTempPartAndReplace(new_data_part, nullptr, out_transaction);
+    auto replaced_parts = data.renameTempPartAndReplace(new_data_part, txn.get(), nullptr, out_transaction);
 
     /// Let's check that all original parts have been deleted and only them.
     if (replaced_parts.size() != parts.size())

src/Storages/MergeTree/MergeTreeDataMergerMutator.h

@@ -40,7 +40,10 @@ enum class ExecuteTTLType
 class MergeTreeDataMergerMutator
 {
 public:
-    using AllowedMergingPredicate = std::function<bool (const MergeTreeData::DataPartPtr &, const MergeTreeData::DataPartPtr &, String *)>;
+    using AllowedMergingPredicate = std::function<bool (const MergeTreeData::DataPartPtr &,
+                                                        const MergeTreeData::DataPartPtr &,
+                                                        const MergeTreeTransaction *,
+                                                        String *)>;
 
     MergeTreeDataMergerMutator(MergeTreeData & data_, size_t max_tasks_count_);
 
@@ -72,6 +75,7 @@ public:
         size_t max_total_size_to_merge,
         const AllowedMergingPredicate & can_merge,
         bool merge_with_ttl_allowed,
+        const MergeTreeTransactionPtr & txn,
         String * out_disable_reason = nullptr);
 
     /** Select all the parts in the specified partition for merge, if possible.
@@ -85,6 +89,7 @@ public:
         const String & partition_id,
         bool final,
         const StorageMetadataPtr & metadata_snapshot,
+        const MergeTreeTransactionPtr & txn,
         String * out_disable_reason = nullptr,
         bool optimize_skip_merged_partitions = false);
 
@@ -107,6 +112,7 @@ public:
         bool deduplicate,
         const Names & deduplicate_by_columns,
         const MergeTreeData::MergingParams & merging_params,
+        const MergeTreeTransactionPtr & txn,
         const IMergeTreeDataPart * parent_part = nullptr,
         const String & suffix = "");
 
@@ -118,12 +124,14 @@ public:
         MergeListEntry * merge_entry,
         time_t time_of_mutation,
         ContextPtr context,
+        const MergeTreeTransactionPtr & txn,
         ReservationSharedPtr space_reservation,
         TableLockHolder & table_lock_holder);
 
     MergeTreeData::DataPartPtr renameMergedTemporaryPart(
         MergeTreeData::MutableDataPartPtr & new_data_part,
         const MergeTreeData::DataPartsVector & parts,
+        const MergeTreeTransactionPtr & txn,
         MergeTreeData::Transaction * out_transaction = nullptr);
 
 

src/Storages/MergeTree/MergeTreeDataPartInMemory.cpp

@@ -91,7 +91,7 @@ void MergeTreeDataPartInMemory::flushToDisk(const String & base_path, const Stri
 
     auto compression_codec = storage.getContext()->chooseCompressionCodec(0, 0);
     auto indices = MergeTreeIndexFactory::instance().getMany(metadata_snapshot->getSecondaryIndices());
-    MergedBlockOutputStream out(new_data_part, metadata_snapshot, columns, indices, compression_codec);
+    MergedBlockOutputStream out(new_data_part, metadata_snapshot, columns, indices, compression_codec, NO_TRANSACTION_PTR);
     out.write(block);
     const auto & projections = metadata_snapshot->getProjections();
     for (const auto & [projection_name, projection] : projection_parts)
@@ -122,7 +122,7 @@ void MergeTreeDataPartInMemory::flushToDisk(const String & base_path, const Stri
             auto projection_indices = MergeTreeIndexFactory::instance().getMany(desc.metadata->getSecondaryIndices());
             MergedBlockOutputStream projection_out(
                 projection_data_part, desc.metadata, projection_part->columns, projection_indices,
-                projection_compression_codec);
+                projection_compression_codec, NO_TRANSACTION_PTR);
 
             projection_out.write(projection_part->block);
             projection_out.finalizePart(projection_data_part, false);

src/Storages/MergeTree/MergeTreeDataSelectExecutor.cpp

@@ -130,9 +130,11 @@ QueryPlanPtr MergeTreeDataSelectExecutor::read(
         return std::make_unique<QueryPlan>();
 
     const auto & settings = context->getSettingsRef();
+
     const auto & metadata_for_reading = storage_snapshot->getMetadataForQuery();
 
     const auto & snapshot_data = assert_cast<const MergeTreeData::SnapshotData &>(*storage_snapshot->data);
+
     const auto & parts = snapshot_data.parts;
 
     if (!query_info.projection)

src/Storages/MergeTree/MergeTreeDataWriter.cpp

@@ -7,6 +7,7 @@
 #include <Interpreters/AggregationCommon.h>
 #include <Interpreters/Context.h>
 #include <Interpreters/InterpreterSelectQuery.h>
+#include <Interpreters/MergeTreeTransaction.h>
 #include <IO/HashingWriteBuffer.h>
 #include <DataTypes/DataTypeDateTime.h>
 #include <DataTypes/DataTypeDate.h>
@@ -433,7 +434,9 @@ MergeTreeDataWriter::TemporaryPart MergeTreeDataWriter::writeTempPart(
 
     const auto & index_factory = MergeTreeIndexFactory::instance();
     auto out = std::make_unique<MergedBlockOutputStream>(new_data_part, metadata_snapshot, columns,
-        index_factory.getMany(metadata_snapshot->getSecondaryIndices()), compression_codec, false, false, context->getWriteSettings());
+        index_factory.getMany(metadata_snapshot->getSecondaryIndices()), compression_codec,
+        context->getCurrentTransaction(), false, false, context->getWriteSettings());
+
 
     out->writeWithPermutation(block, perm_ptr);
 
@@ -565,7 +568,8 @@ MergeTreeDataWriter::TemporaryPart MergeTreeDataWriter::writeProjectionPartImpl(
         metadata_snapshot,
         columns,
         MergeTreeIndices{},
-        compression_codec);
+        compression_codec,
+        NO_TRANSACTION_PTR);
 
     out->writeWithPermutation(block, perm_ptr);
     auto finalizer = out->finalizePartAsync(new_data_part, false);

src/Storages/MergeTree/MergeTreeMutationEntry.cpp

@@ -4,6 +4,7 @@
 #include <IO/WriteBufferFromFile.h>
 #include <IO/ReadBufferFromFile.h>
 #include <IO/ReadBufferFromString.h>
+#include <Interpreters/TransactionLog.h>
 
 #include <utility>
 
@@ -43,13 +44,15 @@ UInt64 MergeTreeMutationEntry::parseFileName(const String & file_name_)
     throw Exception(ErrorCodes::BAD_ARGUMENTS, "Cannot parse mutation version from file name, expected 'mutation_<UInt64>.txt', got '{}'", file_name_);
 }
 
-MergeTreeMutationEntry::MergeTreeMutationEntry(MutationCommands commands_, DiskPtr disk_, const String & path_prefix_, UInt64 tmp_number, const WriteSettings & settings)
+MergeTreeMutationEntry::MergeTreeMutationEntry(MutationCommands commands_, DiskPtr disk_, const String & path_prefix_, UInt64 tmp_number,
+                                               const TransactionID & tid_, const WriteSettings & settings)
     : create_time(time(nullptr))
     , commands(std::move(commands_))
     , disk(std::move(disk_))
     , path_prefix(path_prefix_)
     , file_name("tmp_mutation_" + toString(tmp_number) + ".txt")
     , is_temp(true)
+    , tid(tid_)
 {
     try
     {
@@ -59,6 +62,16 @@ MergeTreeMutationEntry::MergeTreeMutationEntry(MutationCommands commands_, DiskP
         *out << "commands: ";
         commands.writeText(*out);
         *out << "\n";
+        if (tid.isPrehistoric())
+        {
+            csn = Tx::PrehistoricCSN;
+        }
+        else
+        {
+            *out << "tid: ";
+            TransactionID::write(tid, *out);
+            *out << "\n";
+        }
         out->sync();
     }
     catch (...)
@@ -90,6 +103,14 @@ void MergeTreeMutationEntry::removeFile()
     }
 }
 
+void MergeTreeMutationEntry::writeCSN(CSN csn_)
+{
+    csn = csn_;
+    auto out = disk->writeFile(path_prefix + file_name, 256, WriteMode::Append);
+    *out << "csn: " << csn << "\n";
+    out->finalize();
+}
+
 MergeTreeMutationEntry::MergeTreeMutationEntry(DiskPtr disk_, const String & path_prefix_, const String & file_name_)
     : disk(std::move(disk_))
     , path_prefix(path_prefix_)
@@ -111,6 +132,23 @@ MergeTreeMutationEntry::MergeTreeMutationEntry(DiskPtr disk_, const String & pat
     commands.readText(*buf);
     *buf >> "\n";
 
+    if (buf->eof())
+    {
+        tid = Tx::PrehistoricTID;
+        csn = Tx::PrehistoricCSN;
+    }
+    else
+    {
+        *buf >> "tid: ";
+        tid = TransactionID::read(*buf);
+        *buf >> "\n";
+
+        if (!buf->eof())
+        {
+            *buf >> "csn: " >> csn >> "\n";
+        }
+    }
+
     assertEOF(*buf);
 }
 

src/Storages/MergeTree/MergeTreeMutationEntry.h

@@ -4,6 +4,7 @@
 #include <Disks/IDisk.h>
 #include <Storages/MergeTree/MergeTreePartInfo.h>
 #include <Storages/MutationCommands.h>
+#include <Common/TransactionID.h>
 
 
 namespace DB
@@ -28,8 +29,15 @@ struct MergeTreeMutationEntry
     time_t latest_fail_time = 0;
     String latest_fail_reason;
 
+    /// ID of transaction which has created mutation.
+    TransactionID tid = Tx::PrehistoricTID;
+    /// CSN of transaction which has created mutation
+    /// or UnknownCSN if it's not committed (yet) or RolledBackCSN if it's rolled back or PrehistoricCSN if there is no transaction.
+    CSN csn = Tx::UnknownCSN;
+
     /// Create a new entry and write it to a temporary file.
-    MergeTreeMutationEntry(MutationCommands commands_, DiskPtr disk, const String & path_prefix_, UInt64 tmp_number, const WriteSettings & settings);
+    MergeTreeMutationEntry(MutationCommands commands_, DiskPtr disk, const String & path_prefix_, UInt64 tmp_number,
+                           const TransactionID & tid_, const WriteSettings & settings);
     MergeTreeMutationEntry(const MergeTreeMutationEntry &) = delete;
     MergeTreeMutationEntry(MergeTreeMutationEntry &&) = default;
 
@@ -38,6 +46,8 @@ struct MergeTreeMutationEntry
 
     void removeFile();
 
+    void writeCSN(CSN csn_);
+
     static String versionToFileName(UInt64 block_number_);
     static UInt64 tryParseFileName(const String & file_name_);
     static UInt64 parseFileName(const String & file_name_);

src/Storages/MergeTree/MergeTreePartsMover.cpp

@@ -94,7 +94,7 @@ bool MergeTreePartsMover::selectPartsForMove(
     unsigned parts_to_move_by_ttl_rules = 0;
     double parts_to_move_total_size_bytes = 0.0;
 
-    MergeTreeData::DataPartsVector data_parts = data->getDataPartsVector();
+    MergeTreeData::DataPartsVector data_parts = data->getDataPartsVectorForInternalUsage();
 
     if (data_parts.empty())
         return false;
@@ -231,6 +231,7 @@ MergeTreeData::DataPartPtr MergeTreePartsMover::clonePart(const MergeTreeMoveEnt
     LOG_TRACE(log, "Part {} was cloned to {}", part->name, cloned_part->getFullPath());
 
     cloned_part->loadColumnsChecksumsIndexes(true, true);
+    cloned_part->loadVersionMetadata();
     cloned_part->modification_time = disk->getLastModified(cloned_part->getFullRelativePath()).epochTime();
     return cloned_part;
 

src/Storages/MergeTree/MergeTreeSettings.h

@@ -61,6 +61,7 @@ struct Settings;
     M(UInt64, merge_selecting_sleep_ms, 5000, "Sleep time for merge selecting when no part selected, a lower setting will trigger selecting tasks in background_schedule_pool frequently which result in large amount of requests to zookeeper in large-scale clusters", 0) \
     M(UInt64, merge_tree_clear_old_temporary_directories_interval_seconds, 60, "The period of executing the clear old temporary directories operation in background.", 0) \
     M(UInt64, merge_tree_clear_old_parts_interval_seconds, 1, "The period of executing the clear old parts operation in background.", 0) \
+    M(Bool, remove_rolled_back_parts_immediately, 1, "Setting for an incomplete experimental feature.", 0) \
     \
     /** Inserts settings. */ \
     M(UInt64, parts_to_delay_insert, 150, "If table contains at least that many active parts in single partition, artificially slow down insert into table.", 0) \

src/Storages/MergeTree/MergeTreeSink.cpp

@@ -50,7 +50,7 @@ struct MergeTreeSink::DelayedChunk
 void MergeTreeSink::consume(Chunk chunk)
 {
     auto block = getHeader().cloneWithColumns(chunk.detachColumns());
-    auto storage_snapshot = storage.getStorageSnapshot(metadata_snapshot);
+    auto storage_snapshot = storage.getStorageSnapshot(metadata_snapshot, context);
 
     storage.writer.deduceTypesOfObjectColumns(storage_snapshot, block);
     auto part_blocks = storage.writer.splitBlockIntoParts(block, max_parts_per_block, metadata_snapshot, context);
@@ -134,7 +134,7 @@ void MergeTreeSink::finishDelayedChunk()
         auto & part = partition.temp_part.part;
 
         /// Part can be deduplicated, so increment counters and add to part log only if it's really added
-        if (storage.renameTempPartAndAdd(part, &storage.increment, nullptr, storage.getDeduplicationLog(), partition.block_dedup_token))
+        if (storage.renameTempPartAndAdd(part, context->getCurrentTransaction().get(), &storage.increment, nullptr, storage.getDeduplicationLog(), partition.block_dedup_token))
         {
             PartLog::addNewPart(storage.getContext(), part, partition.elapsed_ns);
 

src/Storages/MergeTree/MergeTreeWriteAheadLog.cpp

@@ -197,7 +197,8 @@ MergeTreeData::MutableDataPartsVector MergeTreeWriteAheadLog::restore(const Stor
                 metadata_snapshot,
                 block.getNamesAndTypesList(),
                 {},
-                CompressionCodecFactory::instance().get("NONE", {}));
+                CompressionCodecFactory::instance().get("NONE", {}),
+                NO_TRANSACTION_PTR);
 
             part->minmax_idx->update(block, storage.getMinMaxColumnsNames(metadata_snapshot->getPartitionKey()));
             part->partition.create(metadata_snapshot, block, 0, context);

src/Storages/MergeTree/MergedBlockOutputStream.cpp

@@ -1,5 +1,6 @@
 #include <Storages/MergeTree/MergedBlockOutputStream.h>
 #include <Interpreters/Context.h>
+#include <Interpreters/MergeTreeTransaction.h>
 #include <Parsers/queryToString.h>
 
 
@@ -18,6 +19,7 @@ MergedBlockOutputStream::MergedBlockOutputStream(
     const NamesAndTypesList & columns_list_,
     const MergeTreeIndices & skip_indices,
     CompressionCodecPtr default_codec_,
+    const MergeTreeTransactionPtr & txn,
     bool reset_columns_,
     bool blocks_are_granules_size,
     const WriteSettings & write_settings)
@@ -36,6 +38,13 @@ MergedBlockOutputStream::MergedBlockOutputStream(
     if (!part_path.empty())
         volume->getDisk()->createDirectories(part_path);
 
+    /// We should write version metadata on part creation to distinguish it from parts that were created without transaction.
+    TransactionID tid = txn ? txn->tid : Tx::PrehistoricTID;
+    /// NOTE do not pass context for writing to system.transactions_info_log,
+    /// because part may have temporary name (with temporary block numbers). Will write it later.
+    data_part->version.setCreationTID(tid, nullptr);
+    data_part->storeVersionMetadata();
+
     writer = data_part->getWriter(columns_list, metadata_snapshot, skip_indices, default_codec, writer_settings);
 }
 

src/Storages/MergeTree/MergedBlockOutputStream.h

@@ -20,6 +20,7 @@ public:
         const NamesAndTypesList & columns_list_,
         const MergeTreeIndices & skip_indices,
         CompressionCodecPtr default_codec_,
+        const MergeTreeTransactionPtr & txn,
         bool reset_columns_ = false,
         bool blocks_are_granules_size = false,
         const WriteSettings & write_settings = {});