Instant count() for MergeTree

Use (Replicated)MergeTree's metadata to do trivial count()

dbms/src/Core/Settings.h

@@ -380,6 +380,7 @@ struct Settings : public SettingsCollection<Settings>
     M(SettingUInt64, min_free_disk_space_for_temporary_data, 0, "The minimum disk space to keep while writing temporary data used in external sorting and aggregation.") \
     \
     M(SettingBool, enable_scalar_subquery_optimization, true, "If it is set to true, prevent scalar subqueries from (de)serializing large scalar values and possibly avoid running the same subquery more than once.") \
+    M(SettingBool, optimize_trivial_count_query, true, "Process trivial 'SELECT count() FROM table' query from metadata.") \
     \
     /** Obsolete settings that do nothing but left for compatibility reasons. Remove each one after half a year of obsolescence. */ \
     \

dbms/src/Interpreters/InterpreterSelectQuery.cpp

@@ -26,6 +26,7 @@
 #include <DataStreams/ReverseBlockInputStream.h>
 #include <DataStreams/FillingBlockInputStream.h>
 #include <DataStreams/SquashingBlockInputStream.h>
+#include <DataStreams/OneBlockInputStream.h>
 
 #include <Parsers/ASTFunction.h>
 #include <Parsers/ASTIdentifier.h>
@@ -65,6 +66,7 @@
 #include <Common/checkStackSize.h>
 #include <Parsers/queryToString.h>
 #include <ext/map.h>
+#include <ext/scope_guard.h>
 #include <memory>
 
 #include <Processors/Sources/NullSource.h>
@@ -90,6 +92,7 @@
 #include <Processors/Transforms/FinishSortingTransform.h>
 #include <DataTypes/DataTypeAggregateFunction.h>
 #include <DataStreams/materializeBlock.h>
+#include <IO/MemoryReadWriteBuffer.h>
 
 
 namespace DB
@@ -1273,6 +1276,65 @@ void InterpreterSelectQuery::executeFetchColumns(
     auto & query = getSelectQuery();
     const Settings & settings = context.getSettingsRef();
 
+    /// Optimization for trivial query like SELECT count() FROM table.
+    auto check_trivial_count_query = [&]() -> std::optional<AggregateDescription>
+    {
+        if (!settings.optimize_trivial_count_query || !syntax_analyzer_result->maybe_optimize_trivial_count || !storage
+            || query.sample_size() || query.sample_offset() || query.final() || query.prewhere() || query.where()
+            || !query_analyzer->hasAggregation() || processing_stage != QueryProcessingStage::FetchColumns)
+            return {};
+
+        Names key_names;
+        AggregateDescriptions aggregates;
+        query_analyzer->getAggregateInfo(key_names, aggregates);
+
+        if (aggregates.size() != 1)
+            return {};
+
+        const AggregateDescription & desc = aggregates[0];
+        if (typeid_cast<AggregateFunctionCount *>(desc.function.get()))
+            return desc;
+
+        return {};
+    };
+
+    if (auto desc = check_trivial_count_query())
+    {
+        auto func = desc->function;
+        std::optional<UInt64> num_rows = storage->totalRows();
+        if (num_rows)
+        {
+            AggregateFunctionCount & agg_count = static_cast<AggregateFunctionCount &>(*func);
+
+            /// We will process it up to "WithMergeableState".
+            std::vector<char> state(agg_count.sizeOfData());
+            AggregateDataPtr place = state.data();
+
+            agg_count.create(place);
+            SCOPE_EXIT(agg_count.destroy(place));
+
+            MemoryWriteBuffer out;
+            writeVarUInt(*num_rows, out);
+            auto in = out.tryGetReadBuffer();
+            agg_count.deserialize(place, *in, nullptr);
+
+            auto column = ColumnAggregateFunction::create(func);
+            column->insertFrom(place);
+
+            Block block_with_count{
+                {std::move(column), std::make_shared<DataTypeAggregateFunction>(func, DataTypes(), Array()), desc->column_name}};
+
+            auto istream = std::make_shared<OneBlockInputStream>(block_with_count);
+            if constexpr (pipeline_with_processors)
+                pipeline.init({std::make_shared<SourceFromInputStream>(istream)});
+            else
+                pipeline.streams.emplace_back(istream);
+            from_stage = QueryProcessingStage::WithMergeableState;
+            analysis_result.first_stage = false;
+            return;
+        }
+    }
+
     /// Actions to calculate ALIAS if required.
     ExpressionActionsPtr alias_actions;
 

dbms/src/Interpreters/SyntaxAnalyzer.cpp

@@ -727,6 +727,7 @@ void SyntaxAnalyzerResult::collectUsedColumns(const ASTPtr & query, const NamesA
     /// You need to read at least one column to find the number of rows.
     if (select_query && required.empty())
     {
+        maybe_optimize_trivial_count = true;
         /// We will find a column with minimum <compressed_size, type_size, uncompressed_size>.
         /// Because it is the column that is cheapest to read.
         struct ColumnSizeTuple

dbms/src/Interpreters/SyntaxAnalyzer.h

@@ -48,6 +48,8 @@ struct SyntaxAnalyzerResult
     /// Results of scalar sub queries
     Scalars scalars;
 
+    bool maybe_optimize_trivial_count = false;
+
     void collectUsedColumns(const ASTPtr & query, const NamesAndTypesList & additional_source_columns);
     Names requiredSourceColumns() const { return required_source_columns.getNames(); }
     const Scalars & getScalars() const { return scalars; }

dbms/src/Storages/IStorage.h

@@ -406,6 +406,13 @@ public:
     /// Returns storage policy if storage supports it
     virtual DiskSpace::StoragePolicyPtr getStoragePolicy() const { return {}; }
 
+    /** If it is possible to quickly determine exact number of rows in the table at this moment of time, then return it.
+     */
+    virtual std::optional<UInt64> totalRows() const
+    {
+        return {};
+    }
+
 private:
     /// You always need to take the next three locks in this order.
 

dbms/src/Storages/MergeTree/MergeTreeData.cpp

@@ -2374,6 +2374,20 @@ size_t MergeTreeData::getTotalActiveSizeInBytes() const
 }
 
 
+size_t MergeTreeData::getTotalActiveSizeInRows() const
+{
+    size_t res = 0;
+    {
+        auto lock = lockParts();
+
+        for (auto & part : getDataPartsStateRange(DataPartState::Committed))
+            res += part->rows_count;
+    }
+
+    return res;
+}
+
+
 size_t MergeTreeData::getPartsCount() const
 {
     auto lock = lockParts();
@@ -2486,7 +2500,7 @@ void MergeTreeData::throwInsertIfNeeded() const
 }
 
 MergeTreeData::DataPartPtr MergeTreeData::getActiveContainingPart(
-    const MergeTreePartInfo & part_info, MergeTreeData::DataPartState state, DataPartsLock & /*lock*/)
+    const MergeTreePartInfo & part_info, MergeTreeData::DataPartState state, DataPartsLock & /*lock*/) const
 {
     auto current_state_parts_range = getDataPartsStateRange(state);
 
@@ -2534,13 +2548,13 @@ void MergeTreeData::swapActivePart(MergeTreeData::DataPartPtr part_copy)
 }
 
 
-MergeTreeData::DataPartPtr MergeTreeData::getActiveContainingPart(const MergeTreePartInfo & part_info)
+MergeTreeData::DataPartPtr MergeTreeData::getActiveContainingPart(const MergeTreePartInfo & part_info) const
 {
     auto lock = lockParts();
     return getActiveContainingPart(part_info, DataPartState::Committed, lock);
 }
 
-MergeTreeData::DataPartPtr MergeTreeData::getActiveContainingPart(const String & part_name)
+MergeTreeData::DataPartPtr MergeTreeData::getActiveContainingPart(const String & part_name) const
 {
     auto part_info = MergeTreePartInfo::fromPartName(part_name, format_version);
     return getActiveContainingPart(part_info);

dbms/src/Storages/MergeTree/MergeTreeData.h

@@ -435,9 +435,9 @@ public:
     DataPartsVector getDataPartsVector() const;
 
     /// Returns a committed part with the given name or a part containing it. If there is no such part, returns nullptr.
-    DataPartPtr getActiveContainingPart(const String & part_name);
-    DataPartPtr getActiveContainingPart(const MergeTreePartInfo & part_info);
-    DataPartPtr getActiveContainingPart(const MergeTreePartInfo & part_info, DataPartState state, DataPartsLock & lock);
+    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;
 
     /// Swap part with it's identical copy (possible with another path on another disk).
     /// If original part is not active or doesn't exist exception will be thrown.
@@ -453,6 +453,8 @@ public:
     /// Total size of active parts in bytes.
     size_t getTotalActiveSizeInBytes() const;
 
+    size_t getTotalActiveSizeInRows() const;
+
     size_t getPartsCount() const;
     size_t getMaxPartsCountForPartition() const;
 

dbms/src/Storages/StorageMergeTree.cpp

@@ -134,6 +134,11 @@ BlockInputStreams StorageMergeTree::read(
     return reader.read(column_names, query_info, context, max_block_size, num_streams);
 }
 
+std::optional<UInt64> StorageMergeTree::totalRows() const
+{
+    return getTotalActiveSizeInRows();
+}
+
 BlockOutputStreamPtr StorageMergeTree::write(const ASTPtr & /*query*/, const Context & context)
 {
     return std::make_shared<MergeTreeBlockOutputStream>(*this, context.getSettingsRef().max_partitions_per_insert_block);

dbms/src/Storages/StorageMergeTree.h

@@ -45,6 +45,8 @@ public:
         size_t max_block_size,
         unsigned num_streams) override;
 
+    std::optional<UInt64> totalRows() const override;
+
     BlockOutputStreamPtr write(const ASTPtr & query, const Context & context) override;
 
     /** Perform the next step in combining the parts.

dbms/src/Storages/StorageReplicatedMergeTree.cpp

@@ -171,13 +171,13 @@ void StorageReplicatedMergeTree::setZooKeeper(zkutil::ZooKeeperPtr zookeeper)
     current_zookeeper = zookeeper;
 }
 
-zkutil::ZooKeeperPtr StorageReplicatedMergeTree::tryGetZooKeeper()
+zkutil::ZooKeeperPtr StorageReplicatedMergeTree::tryGetZooKeeper() const
 {
     std::lock_guard lock(current_zookeeper_mutex);
     return current_zookeeper;
 }
 
-zkutil::ZooKeeperPtr StorageReplicatedMergeTree::getZooKeeper()
+zkutil::ZooKeeperPtr StorageReplicatedMergeTree::getZooKeeper() const
 {
     auto res = tryGetZooKeeper();
     if (!res)
@@ -2920,28 +2920,14 @@ StorageReplicatedMergeTree::~StorageReplicatedMergeTree()
 }
 
 
-BlockInputStreams StorageReplicatedMergeTree::read(
-    const Names & column_names,
-    const SelectQueryInfo & query_info,
-    const Context & context,
-    QueryProcessingStage::Enum /*processed_stage*/,
-    const size_t max_block_size,
-    const unsigned num_streams)
-{
-    const Settings & settings_ = context.getSettingsRef();
-
-    /** The `select_sequential_consistency` setting has two meanings:
-    * 1. To throw an exception if on a replica there are not all parts which have been written down on quorum of remaining replicas.
-    * 2. Do not read parts that have not yet been written to the quorum of the replicas.
-    * For this you have to synchronously go to ZooKeeper.
-    */
-    if (settings_.select_sequential_consistency)
+ReplicatedMergeTreeQuorumAddedParts::PartitionIdToMaxBlock StorageReplicatedMergeTree::getMaxAddedBlocks() const
 {
     ReplicatedMergeTreeQuorumAddedParts::PartitionIdToMaxBlock max_added_blocks;
 
     for (const auto & data_part : getDataParts())
     {
-            max_added_blocks[data_part->info.partition_id] = std::max(max_added_blocks[data_part->info.partition_id], data_part->info.max_block);
+        max_added_blocks[data_part->info.partition_id]
+            = std::max(max_added_blocks[data_part->info.partition_id], data_part->info.max_block);
     }
 
     auto zookeeper = getZooKeeper();
@@ -2973,14 +2959,37 @@ BlockInputStreams StorageReplicatedMergeTree::read(
 
             for (const auto & added_part : added_parts)
                 if (!getActiveContainingPart(added_part.second))
-                        throw Exception("Replica doesn't have part " + added_part.second + " which was successfully written to quorum of other replicas."
-                            " Send query to another replica or disable 'select_sequential_consistency' setting.", ErrorCodes::REPLICA_IS_NOT_IN_QUORUM);
+                    throw Exception(
+                        "Replica doesn't have part " + added_part.second
+                            + " which was successfully written to quorum of other replicas."
+                              " Send query to another replica or disable 'select_sequential_consistency' setting.",
+                        ErrorCodes::REPLICA_IS_NOT_IN_QUORUM);
 
             for (const auto & max_block : part_with_quorum.getMaxInsertedBlocks())
                 max_added_blocks[max_block.first] = max_block.second;
         }
     }
+    return max_added_blocks;
+}
 
+BlockInputStreams StorageReplicatedMergeTree::read(
+    const Names & column_names,
+    const SelectQueryInfo & query_info,
+    const Context & context,
+    QueryProcessingStage::Enum /*processed_stage*/,
+    const size_t max_block_size,
+    const unsigned num_streams)
+{
+    const Settings & settings_ = context.getSettingsRef();
+
+    /** The `select_sequential_consistency` setting has two meanings:
+    * 1. To throw an exception if on a replica there are not all parts which have been written down on quorum of remaining replicas.
+    * 2. Do not read parts that have not yet been written to the quorum of the replicas.
+    * For this you have to synchronously go to ZooKeeper.
+    */
+    if (settings_.select_sequential_consistency)
+    {
+        auto max_added_blocks = getMaxAddedBlocks();
         return reader.read(column_names, query_info, context, max_block_size, num_streams, &max_added_blocks);
     }
 
@@ -2988,6 +2997,26 @@ BlockInputStreams StorageReplicatedMergeTree::read(
 }
 
 
+std::optional<UInt64> StorageReplicatedMergeTree::totalRows() const
+{
+    size_t res = 0;
+    auto max_added_blocks = getMaxAddedBlocks();
+    auto lock = lockParts();
+    for (auto & part : getDataPartsStateRange(DataPartState::Committed))
+    {
+        if (part->isEmpty())
+            continue;
+
+        auto blocks_iterator = max_added_blocks.find(part->info.partition_id);
+        if (blocks_iterator == max_added_blocks.end() || part->info.max_block > blocks_iterator->second)
+            continue;
+
+        res += part->rows_count;
+    }
+    return res;
+}
+
+
 void StorageReplicatedMergeTree::assertNotReadonly() const
 {
     if (is_readonly)

dbms/src/Storages/StorageReplicatedMergeTree.h

@@ -96,6 +96,8 @@ public:
         size_t max_block_size,
         unsigned num_streams) override;
 
+    std::optional<UInt64> totalRows() const override;
+
     BlockOutputStreamPtr write(const ASTPtr & query, const Context & context) override;
 
     bool optimize(const ASTPtr & query, const ASTPtr & partition, bool final, bool deduplicate, const Context & query_context) override;
@@ -174,6 +176,10 @@ public:
     bool canUseAdaptiveGranularity() const override;
 
 private:
+
+    /// Get a sequential consistent view of current parts.
+    ReplicatedMergeTreeQuorumAddedParts::PartitionIdToMaxBlock getMaxAddedBlocks() const;
+
     /// Delete old parts from disk and from ZooKeeper.
     void clearOldPartsAndRemoveFromZK();
 
@@ -191,10 +197,10 @@ private:
     using LogEntryPtr = LogEntry::Ptr;
 
     zkutil::ZooKeeperPtr current_zookeeper;        /// Use only the methods below.
-    std::mutex current_zookeeper_mutex;            /// To recreate the session in the background thread.
+    mutable std::mutex current_zookeeper_mutex;    /// To recreate the session in the background thread.
 
-    zkutil::ZooKeeperPtr tryGetZooKeeper();
-    zkutil::ZooKeeperPtr getZooKeeper();
+    zkutil::ZooKeeperPtr tryGetZooKeeper() const;
+    zkutil::ZooKeeperPtr getZooKeeper() const;
     void setZooKeeper(zkutil::ZooKeeperPtr zookeeper);
 
     /// If true, the table is offline and can not be written to it.

dbms/tests/queries/0_stateless/00971_merge_tree_uniform_read_distribution_and_max_rows_to_read.sql

@@ -14,9 +14,9 @@ SELECT count() FROM merge_tree;
 SET max_rows_to_read = 900000;
 
 SET merge_tree_uniform_read_distribution = 1;
-SELECT count() FROM merge_tree; -- { serverError 158 }
+SELECT count() FROM merge_tree WHERE not ignore(); -- { serverError 158 }
 
 SET merge_tree_uniform_read_distribution = 0;
-SELECT count() FROM merge_tree; -- { serverError 158 }
+SELECT count() FROM merge_tree WHERE not ignore(); -- { serverError 158 }
 
 DROP TABLE merge_tree;