ClickHouse/src/Storages/IStorage.h
2023-04-24 16:13:27 +02:00

683 lines
30 KiB
C++

#pragma once
#include <Core/Names.h>
#include <Core/QueryProcessingStage.h>
#include <Databases/IDatabase.h>
#include <Interpreters/CancellationCode.h>
#include <Interpreters/Context_fwd.h>
#include <Interpreters/StorageID.h>
#include <Storages/CheckResults.h>
#include <Storages/ColumnDependency.h>
#include <Storages/IStorage_fwd.h>
#include <Storages/SelectQueryDescription.h>
#include <Storages/StorageInMemoryMetadata.h>
#include <Storages/TableLockHolder.h>
#include <Storages/StorageSnapshot.h>
#include <Common/ActionLock.h>
#include <Common/Exception.h>
#include <Common/RWLock.h>
#include <Common/TypePromotion.h>
#include <optional>
#include <compare>
namespace DB
{
namespace ErrorCodes
{
extern const int NOT_IMPLEMENTED;
}
using StorageActionBlockType = size_t;
class ASTCreateQuery;
class ASTInsertQuery;
struct Settings;
class AlterCommands;
class MutationCommands;
struct PartitionCommand;
using PartitionCommands = std::vector<PartitionCommand>;
class IProcessor;
using ProcessorPtr = std::shared_ptr<IProcessor>;
using Processors = std::vector<ProcessorPtr>;
class Pipe;
class QueryPlan;
using QueryPlanPtr = std::unique_ptr<QueryPlan>;
class SinkToStorage;
using SinkToStoragePtr = std::shared_ptr<SinkToStorage>;
class QueryPipeline;
class IStoragePolicy;
using StoragePolicyPtr = std::shared_ptr<const IStoragePolicy>;
struct StreamLocalLimits;
class EnabledQuota;
struct SelectQueryInfo;
using NameDependencies = std::unordered_map<String, std::vector<String>>;
using DatabaseAndTableName = std::pair<String, String>;
class BackupEntriesCollector;
class RestorerFromBackup;
struct ColumnSize
{
size_t marks = 0;
size_t data_compressed = 0;
size_t data_uncompressed = 0;
void add(const ColumnSize & other)
{
marks += other.marks;
data_compressed += other.data_compressed;
data_uncompressed += other.data_uncompressed;
}
};
using IndexSize = ColumnSize;
/** Storage. Describes the table. Responsible for
* - storage of the table data;
* - the definition in which files (or not in files) the data is stored;
* - data lookups and appends;
* - data storage structure (compression, etc.)
* - concurrent access to data (locks, etc.)
*/
class IStorage : public std::enable_shared_from_this<IStorage>, public TypePromotion<IStorage>, public IHints<1, IStorage>
{
public:
IStorage() = delete;
/// Storage metadata can be set separately in setInMemoryMetadata method
explicit IStorage(StorageID storage_id_)
: storage_id(std::move(storage_id_))
, metadata(std::make_unique<StorageInMemoryMetadata>()) {}
IStorage(const IStorage &) = delete;
IStorage & operator=(const IStorage &) = delete;
/// The main name of the table type (for example, StorageMergeTree).
virtual std::string getName() const = 0;
/// The name of the table.
StorageID getStorageID() const;
virtual bool isMergeTree() const { return false; }
/// Returns true if the storage receives data from a remote server or servers.
virtual bool isRemote() const { return false; }
/// Returns true if the storage is a view of a table or another view.
virtual bool isView() const { return false; }
/// Returns true if the storage is dictionary
virtual bool isDictionary() const { return false; }
/// Returns true if the storage supports queries with the SAMPLE section.
virtual bool supportsSampling() const { return getInMemoryMetadataPtr()->hasSamplingKey(); }
/// Returns true if the storage supports queries with the FINAL section.
virtual bool supportsFinal() const { return false; }
/// Returns true if the storage supports insert queries with the PARTITION BY section.
virtual bool supportsPartitionBy() const { return false; }
/// Returns true if the storage supports queries with the TTL section.
virtual bool supportsTTL() const { return false; }
/// Returns true if the storage supports queries with the PREWHERE section.
virtual bool supportsPrewhere() const { return false; }
/// Returns which columns supports PREWHERE, or empty std::nullopt if all columns is supported.
/// This is needed for engines whose aggregates data from multiple tables, like Merge.
virtual std::optional<NameSet> supportedPrewhereColumns() const { return std::nullopt; }
/// Returns true if the storage supports optimization of moving conditions to PREWHERE section.
virtual bool canMoveConditionsToPrewhere() const { return supportsPrewhere(); }
/// Returns true if the storage replicates SELECT, INSERT and ALTER commands among replicas.
virtual bool supportsReplication() const { return false; }
/// Returns true if the storage supports parallel insert.
virtual bool supportsParallelInsert() const { return false; }
/// Returns true if the storage supports deduplication of inserted data blocks.
virtual bool supportsDeduplication() const { return false; }
/// Returns true if the blocks shouldn't be pushed to associated views on insert.
virtual bool noPushingToViews() const { return false; }
/// Read query returns streams which automatically distribute data between themselves.
/// So, it's impossible for one stream run out of data when there is data in other streams.
/// Example is StorageSystemNumbers.
virtual bool hasEvenlyDistributedRead() const { return false; }
/// 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; }
/// Requires squashing small blocks to large for optimal storage.
/// This is true for most storages that store data on disk.
virtual bool prefersLargeBlocks() const { return true; }
/// Returns true if the storage is for system, which cannot be target of SHOW CREATE TABLE.
virtual bool isSystemStorage() const { return false; }
/// Optional size information of each physical column.
/// Currently it's only used by the MergeTree family for query optimizations.
using ColumnSizeByName = std::unordered_map<std::string, ColumnSize>;
virtual ColumnSizeByName getColumnSizes() const { return {}; }
/// Optional size information of each secondary index.
/// Valid only for MergeTree family.
using IndexSizeByName = std::unordered_map<std::string, IndexSize>;
virtual IndexSizeByName getSecondaryIndexSizes() const { return {}; }
/// Get mutable version (snapshot) of storage metadata. Metadata object is
/// multiversion, so it can be concurrently changed, but returned copy can be
/// used without any locks.
StorageInMemoryMetadata getInMemoryMetadata() const { return *metadata.get(); }
/// Get immutable version (snapshot) of storage metadata. Metadata object is
/// multiversion, so it can be concurrently changed, but returned copy can be
/// used without any locks.
StorageMetadataPtr getInMemoryMetadataPtr() const { return metadata.get(); }
/// Update storage metadata. Used in ALTER or initialization of Storage.
/// Metadata object is multiversion, so this method can be called without
/// any locks.
void setInMemoryMetadata(const StorageInMemoryMetadata & metadata_)
{
metadata.set(std::make_unique<StorageInMemoryMetadata>(metadata_));
}
/// Return list of virtual columns (like _part, _table, etc). In the vast
/// majority of cases virtual columns are static constant part of Storage
/// class and don't depend on Storage object. But sometimes we have fake
/// storages, like Merge, which works as proxy for other storages and it's
/// virtual columns must contain virtual columns from underlying table.
///
/// User can create columns with the same name as virtual column. After that
/// virtual column will be overridden and inaccessible.
///
/// By default return empty list of columns.
virtual NamesAndTypesList getVirtuals() const;
Names getAllRegisteredNames() const override;
NameDependencies getDependentViewsByColumn(ContextPtr context) const;
/// Returns whether the column is virtual - by default all columns are real.
/// Initially reserved virtual column name may be shadowed by real column.
bool isVirtualColumn(const String & column_name, const StorageMetadataPtr & metadata_snapshot) const;
/// Modify a CREATE TABLE query to make a variant which must be written to a backup.
virtual void adjustCreateQueryForBackup(ASTPtr & create_query) const;
/// Makes backup entries to backup the data of this storage.
virtual void backupData(BackupEntriesCollector & backup_entries_collector, const String & data_path_in_backup, const std::optional<ASTs> & partitions);
/// Extracts data from the backup and put it to the storage.
virtual void restoreDataFromBackup(RestorerFromBackup & restorer, const String & data_path_in_backup, const std::optional<ASTs> & partitions);
/// Returns true if the storage supports backup/restore for specific partitions.
virtual bool supportsBackupPartition() const { return false; }
/// Return true if there is at least one part containing lightweight deleted mask.
virtual bool hasLightweightDeletedMask() const { return false; }
/// Return true if storage can execute lightweight delete mutations.
virtual bool supportsLightweightDelete() const { return false; }
/// Return true if storage can execute 'DELETE FROM' mutations. This is different from lightweight delete
/// because those are internally translated into 'ALTER UDPATE' mutations.
virtual bool supportsDelete() const { return false; }
private:
StorageID storage_id;
mutable std::mutex id_mutex;
/// Multiversion storage metadata. Allows to read/write storage metadata
/// without locks.
MultiVersionStorageMetadataPtr metadata;
protected:
RWLockImpl::LockHolder tryLockTimed(
const RWLock & rwlock, RWLockImpl::Type type, const String & query_id, const std::chrono::milliseconds & acquire_timeout) const;
public:
/// Lock table for share. This lock must be acquired if you want to be sure,
/// that table will be not dropped while you holding this lock. It's used in
/// variety of cases starting from SELECT queries to background merges in
/// MergeTree.
TableLockHolder lockForShare(const String & query_id, const std::chrono::milliseconds & acquire_timeout);
/// Similar to lockForShare, but returns a nullptr if the table is dropped while
/// acquiring the lock instead of raising a TABLE_IS_DROPPED exception
TableLockHolder tryLockForShare(const String & query_id, const std::chrono::milliseconds & acquire_timeout);
/// Lock table for alter. This lock must be acquired in ALTER queries to be
/// sure, that we execute only one simultaneous alter. Doesn't affect share lock.
using AlterLockHolder = std::unique_lock<std::timed_mutex>;
AlterLockHolder lockForAlter(const std::chrono::milliseconds & acquire_timeout);
/// Lock table exclusively. This lock must be acquired if you want to be
/// sure, that no other thread (SELECT, merge, ALTER, etc.) doing something
/// with table. For example it allows to wait all threads before DROP or
/// truncate query.
///
/// NOTE: You have to be 100% sure that you need this lock. It's extremely
/// heavyweight and makes table irresponsive.
TableExclusiveLockHolder lockExclusively(const String & query_id, const std::chrono::milliseconds & acquire_timeout);
/** Returns stage to which query is going to be processed in read() function.
* (Normally, the function only reads the columns from the list, but in other cases,
* for example, the request can be partially processed on a remote server, or an aggregate projection.)
*
* SelectQueryInfo is required since the stage can depends on the query
* (see Distributed() engine and optimize_skip_unused_shards,
* see also MergeTree engine and projection optimization).
* And to store optimized cluster (after optimize_skip_unused_shards).
* It will also store needed stuff for projection query pipeline.
*
* QueryProcessingStage::Enum required for Distributed over Distributed,
* since it cannot return Complete for intermediate queries never.
*/
virtual QueryProcessingStage::Enum getQueryProcessingStage(ContextPtr, QueryProcessingStage::Enum, const StorageSnapshotPtr &, SelectQueryInfo &) const
{
return QueryProcessingStage::FetchColumns;
}
/** Watch live changes to the table.
* Accepts a list of columns to read, as well as a description of the query,
* from which information can be extracted about how to retrieve data
* (indexes, locks, etc.)
* Returns a stream with which you can read data sequentially
* or multiple streams for parallel data reading.
* The `processed_stage` info is also written to what stage the request was processed.
* (Normally, the function only reads the columns from the list, but in other cases,
* for example, the request can be partially processed on a remote server.)
*
* context contains settings for one query.
* Usually Storage does not care about these settings, since they are used in the interpreter.
* But, for example, for distributed query processing, the settings are passed to the remote server.
*
* num_streams - a recommendation, how many streams to return,
* if the storage can return a different number of streams.
*
* It is guaranteed that the structure of the table will not change over the lifetime of the returned streams (that is, there will not be ALTER, RENAME and DROP).
*/
virtual Pipe watch(
const Names & /*column_names*/,
const SelectQueryInfo & /*query_info*/,
ContextPtr /*context*/,
QueryProcessingStage::Enum & /*processed_stage*/,
size_t /*max_block_size*/,
size_t /*num_streams*/);
/// Returns true if FINAL modifier must be added to SELECT query depending on required columns.
/// It's needed for ReplacingMergeTree wrappers such as MaterializedMySQL and MaterializedPostrgeSQL
virtual bool needRewriteQueryWithFinal(const Names & /*column_names*/) const { return false; }
private:
/** Read a set of columns from the table.
* Accepts a list of columns to read, as well as a description of the query,
* from which information can be extracted about how to retrieve data
* (indexes, locks, etc.)
* Returns a stream with which you can read data sequentially
* or multiple streams for parallel data reading.
* The `processed_stage` must be the result of getQueryProcessingStage() function.
*
* context contains settings for one query.
* Usually Storage does not care about these settings, since they are used in the interpreter.
* But, for example, for distributed query processing, the settings are passed to the remote server.
*
* num_streams - a recommendation, how many streams to return,
* if the storage can return a different number of streams.
*
* metadata_snapshot is consistent snapshot of table metadata, it should be
* passed in all parts of the returned pipeline. Storage metadata can be
* changed during lifetime of the returned pipeline, but the snapshot is
* guaranteed to be immutable.
*/
virtual Pipe read(
const Names & /*column_names*/,
const StorageSnapshotPtr & /*storage_snapshot*/,
SelectQueryInfo & /*query_info*/,
ContextPtr /*context*/,
QueryProcessingStage::Enum /*processed_stage*/,
size_t /*max_block_size*/,
size_t /*num_streams*/);
/// Should we process blocks of data returned by the storage in parallel
/// even when the storage returned only one stream of data for reading?
/// It is beneficial, for example, when you read from a file quickly,
/// but then do heavy computations on returned blocks.
/// This is enabled by default, but in some cases shouldn't be done.
/// For example, when you read from system.numbers instead of system.numbers_mt,
/// you still expect the data to be processed sequentially.
virtual bool parallelizeOutputAfterReading() const { return true; }
public:
/// Other version of read which adds reading step to query plan.
/// Default implementation creates ReadFromStorageStep and uses usual read.
virtual void read(
QueryPlan & query_plan,
const Names & /*column_names*/,
const StorageSnapshotPtr & /*storage_snapshot*/,
SelectQueryInfo & /*query_info*/,
ContextPtr /*context*/,
QueryProcessingStage::Enum /*processed_stage*/,
size_t /*max_block_size*/,
size_t /*num_streams*/);
/** Writes the data to a table.
* Receives a description of the query, which can contain information about the data write method.
* Returns an object by which you can write data sequentially.
*
* metadata_snapshot is consistent snapshot of table metadata, it should be
* passed in all parts of the returned streams. Storage metadata can be
* changed during lifetime of the returned streams, but the snapshot is
* guaranteed to be immutable.
*/
virtual SinkToStoragePtr write(
const ASTPtr & /*query*/,
const StorageMetadataPtr & /*metadata_snapshot*/,
ContextPtr /*context*/)
{
throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Method write is not supported by storage {}", getName());
}
/** Writes the data to a table in distributed manner.
* It is supposed that implementation looks into SELECT part of the query and executes distributed
* INSERT SELECT if it is possible with current storage as a receiver and query SELECT part as a producer.
*
* Returns query pipeline if distributed writing is possible, and nullptr otherwise.
*/
virtual std::optional<QueryPipeline> distributedWrite(
const ASTInsertQuery & /*query*/,
ContextPtr /*context*/);
/** Delete the table data. Called before deleting the directory with the data.
* The method can be called only after detaching table from Context (when no queries are performed with table).
* The table is not usable during and after call to this method.
* If some queries may still use the table, then it must be called under exclusive lock.
* If you do not need any action other than deleting the directory with data, you can leave this method blank.
*/
virtual void drop() {}
virtual void dropInnerTableIfAny(bool /* sync */, ContextPtr /* context */) {}
/** Clear the table data and leave it empty.
* Must be called under exclusive lock (lockExclusively).
*/
virtual void truncate(
const ASTPtr & /*query*/,
const StorageMetadataPtr & /* metadata_snapshot */,
ContextPtr /* context */,
TableExclusiveLockHolder &)
{
throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Truncate is not supported by storage {}", getName());
}
virtual void checkTableCanBeRenamed(const StorageID & /*new_name*/) const {}
/** Rename the table.
* Renaming a name in a file with metadata, the name in the list of tables in the RAM, is done separately.
* In this function, you need to rename the directory with the data, if any.
* Called when the table structure is locked for write.
* Table UUID must remain unchanged, unless table moved between Ordinary and Atomic databases.
*/
virtual void rename(const String & /*new_path_to_table_data*/, const StorageID & new_table_id)
{
renameInMemory(new_table_id);
}
/**
* Just updates names of database and table without moving any data on disk
* Can be called directly only from DatabaseAtomic.
*/
virtual void renameInMemory(const StorageID & new_table_id);
/** ALTER tables in the form of column changes that do not affect the change
* to Storage or its parameters. Executes under alter lock (lockForAlter).
*/
virtual void alter(const AlterCommands & params, ContextPtr context, AlterLockHolder & alter_lock_holder);
/** Checks that alter commands can be applied to storage. For example, columns can be modified,
* or primary key can be changes, etc.
*/
virtual void checkAlterIsPossible(const AlterCommands & commands, ContextPtr context) const;
/**
* Checks that mutation commands can be applied to storage.
*/
virtual void checkMutationIsPossible(const MutationCommands & commands, const Settings & settings) const;
/** ALTER tables with regard to its partitions.
* Should handle locks for each command on its own.
*/
virtual Pipe alterPartition(
const StorageMetadataPtr & /* metadata_snapshot */,
const PartitionCommands & /* commands */,
ContextPtr /* context */);
/// Checks that partition commands can be applied to storage.
virtual void checkAlterPartitionIsPossible(const PartitionCommands & commands, const StorageMetadataPtr & metadata_snapshot, const Settings & settings) const;
/** Perform any background work. For example, combining parts in a MergeTree type table.
* Returns whether any work has been done.
*/
virtual bool optimize(
const ASTPtr & /*query*/,
const StorageMetadataPtr & /*metadata_snapshot*/,
const ASTPtr & /*partition*/,
bool /*final*/,
bool /*deduplicate*/,
const Names & /* deduplicate_by_columns */,
bool /*cleanup*/,
ContextPtr /*context*/)
{
throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Method optimize is not supported by storage {}", getName());
}
/// Mutate the table contents
virtual void mutate(const MutationCommands &, ContextPtr)
{
throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Mutations are not supported by storage {}", getName());
}
/// Cancel a mutation.
virtual CancellationCode killMutation(const String & /*mutation_id*/)
{
throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Mutations are not supported by storage {}", getName());
}
virtual void waitForMutation(const String & /*mutation_id*/, bool /*wait_for_another_mutation*/)
{
throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Mutations are not supported by storage {}", getName());
}
virtual void setMutationCSN(const String & /*mutation_id*/, UInt64 /*csn*/)
{
throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Mutations are not supported by storage {}", getName());
}
/// Cancel a part move to shard.
virtual CancellationCode killPartMoveToShard(const UUID & /*task_uuid*/)
{
throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Part moves between shards are not supported by storage {}", getName());
}
/** If the table have to do some complicated work on startup,
* that must be postponed after creation of table object
* (like launching some background threads),
* do it in this method.
* You should call this method after creation of object.
* By default, does nothing.
* Cannot be called simultaneously by multiple threads.
*/
virtual void startup() {}
/**
* If the storage requires some complicated work on destroying,
* then you have two virtual methods:
* - flush()
* - shutdown()
*
* @see shutdown()
* @see flush()
*/
void flushAndShutdown()
{
flush();
shutdown();
}
/** If the table have to do some complicated work when destroying an object - do it in advance.
* For example, if the table contains any threads for background work - ask them to complete and wait for completion.
* By default, does nothing.
* Can be called simultaneously from different threads, even after a call to drop().
*/
virtual void shutdown() {}
/// Called before shutdown() to flush data to underlying storage
/// Data in memory need to be persistent
virtual void flush() {}
/// Asks table to stop executing some action identified by action_type
/// If table does not support such type of lock, and empty lock is returned
virtual ActionLock getActionLock(StorageActionBlockType /* action_type */)
{
return {};
}
/// Call when lock from previous method removed
virtual void onActionLockRemove(StorageActionBlockType /* action_type */) {}
std::atomic<bool> is_dropped{false};
std::atomic<bool> is_detached{false};
/// Does table support index for IN sections
virtual bool supportsIndexForIn() const { return false; }
/// Provides a hint that the storage engine may evaluate the IN-condition by using an index.
virtual bool mayBenefitFromIndexForIn(const ASTPtr & /* left_in_operand */, ContextPtr /* query_context */, const StorageMetadataPtr & /* metadata_snapshot */) const { return false; }
/// Checks validity of the data
virtual CheckResults checkData(const ASTPtr & /* query */, ContextPtr /* context */) { throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Check query is not supported for {} storage", getName()); }
/// Checks that table 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 checkTableCanBeDropped() const {}
/// Similar to above but checks for DETACH. It's only used for DICTIONARIES.
virtual void checkTableCanBeDetached() const {}
/// Returns true if Storage may store some data on disk.
/// NOTE: may not be equivalent to !getDataPaths().empty()
virtual bool storesDataOnDisk() const { return false; }
/// Returns data paths if storage supports it, empty vector otherwise.
virtual Strings getDataPaths() const { return {}; }
/// Returns storage policy if storage supports it.
virtual StoragePolicyPtr getStoragePolicy() const { return {}; }
/// Returns true if all disks of storage are read-only or write-once.
/// NOTE: write-once also does not support INSERTs/merges/... for MergeTree
virtual bool isStaticStorage() const;
virtual bool supportsSubsetOfColumns() const { return false; }
/// If it is possible to quickly determine exact number of rows in the table at this moment of time, then return it.
/// Used for:
/// - Simple count() optimization
/// - For total_rows column in system.tables
///
/// Does takes underlying Storage (if any) into account.
virtual std::optional<UInt64> totalRows(const Settings &) const { return {}; }
/// Same as above but also take partition predicate into account.
virtual std::optional<UInt64> totalRowsByPartitionPredicate(const SelectQueryInfo &, ContextPtr) const { return {}; }
/// If it is possible to quickly determine exact number of bytes for the table on storage:
/// - memory (approximated, resident)
/// - disk (compressed)
///
/// Used for:
/// - For total_bytes column in system.tables
//
/// Does not takes underlying Storage (if any) into account
/// (since for Buffer we still need to know how much bytes it uses).
///
/// Memory part should be estimated as a resident memory size.
/// In particular, alloctedBytes() is preferable over bytes()
/// when considering in-memory blocks.
virtual std::optional<UInt64> totalBytes(const Settings &) const { return {}; }
/// Number of rows INSERTed since server start.
///
/// Does not take the underlying Storage (if any) into account.
virtual std::optional<UInt64> lifetimeRows() const { return {}; }
/// Number of bytes INSERTed since server start.
///
/// Does not take the underlying Storage (if any) into account.
virtual std::optional<UInt64> lifetimeBytes() const { return {}; }
/// Creates a storage snapshot from given metadata.
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*/, ContextPtr query_context) const
{
return getStorageSnapshot(metadata_snapshot, query_context);
}
/// A helper to implement read()
static void readFromPipe(
QueryPlan & query_plan,
Pipe pipe,
const Names & column_names,
const StorageSnapshotPtr & storage_snapshot,
SelectQueryInfo & query_info,
ContextPtr context,
std::string storage_name);
private:
/// Lock required for alter queries (lockForAlter).
/// Allows to execute only one simultaneous alter query.
mutable std::timed_mutex alter_lock;
/// Lock required for drop queries. Every thread that want to ensure, that
/// table is not dropped have to table this lock for read (lockForShare).
/// DROP-like queries take this lock for write (lockExclusively), to be sure
/// that all table threads finished.
mutable RWLock drop_lock = RWLockImpl::create();
};
}