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101 lines
3.7 KiB
C++
101 lines
3.7 KiB
C++
#pragma once
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#include <mutex>
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#include <ext/shared_ptr_helper.h>
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#include <Core/NamesAndTypes.h>
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#include <Storages/IStorage.h>
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#include <DataStreams/IBlockOutputStream.h>
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namespace DB
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{
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/** Implements storage in the RAM.
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* Suitable for temporary data.
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* It does not support keys.
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* Data is stored as a set of blocks and is not stored anywhere else.
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*/
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class StorageMemory final : public ext::shared_ptr_helper<StorageMemory>, public IStorage
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{
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friend class MemoryBlockInputStream;
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friend class MemoryBlockOutputStream;
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friend struct ext::shared_ptr_helper<StorageMemory>;
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public:
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String getName() const override { return "Memory"; }
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size_t getSize() const { return data.size(); }
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Pipe read(
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const Names & column_names,
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const StorageMetadataPtr & /*metadata_snapshot*/,
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const SelectQueryInfo & query_info,
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const Context & context,
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QueryProcessingStage::Enum processed_stage,
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size_t max_block_size,
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unsigned num_streams) override;
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bool supportsParallelInsert() const override { return true; }
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BlockOutputStreamPtr write(const ASTPtr & query, const StorageMetadataPtr & metadata_snapshot, const Context & context) override;
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void drop() override;
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void truncate(const ASTPtr &, const StorageMetadataPtr &, const Context &, TableExclusiveLockHolder &) override;
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std::optional<UInt64> totalRows() const override;
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std::optional<UInt64> totalBytes() const override;
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/** Delays initialization of StorageMemory::read() until the first read is actually happen.
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* Usually, fore code like this:
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*
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* auto out = StorageMemory::write();
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* auto in = StorageMemory::read();
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* out->write(new_data);
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*
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* `new_data` won't appear into `in`.
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* However, if delayReadForGlobalSubqueries is called, first read from `in` will check for new_data and return it.
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*
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*
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* Why is delayReadForGlobalSubqueries needed?
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*
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* The fact is that when processing a query of the form
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* SELECT ... FROM remote_test WHERE column GLOBAL IN (subquery),
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* if the distributed remote_test table contains localhost as one of the servers,
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* the query will be interpreted locally again (and not sent over TCP, as in the case of a remote server).
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*
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* The query execution pipeline will be:
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* CreatingSets
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* subquery execution, filling the temporary table with _data1 (1)
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* CreatingSets
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* reading from the table _data1, creating the set (2)
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* read from the table subordinate to remote_test.
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*
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* (The second part of the pipeline under CreateSets is a reinterpretation of the query inside StorageDistributed,
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* the query differs in that the database name and tables are replaced with subordinates, and the subquery is replaced with _data1.)
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*
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* But when creating the pipeline, when creating the source (2), it will be found that the _data1 table is empty
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* (because the query has not started yet), and empty source will be returned as the source.
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* And then, when the query is executed, an empty set will be created in step (2).
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*
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* Therefore, we make the initialization of step (2) delayed
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* - so that it does not occur until step (1) is completed, on which the table will be populated.
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*/
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void delayReadForGlobalSubqueries() { delay_read_for_global_subqueries = true; }
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private:
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/// The data itself. `list` - so that when inserted to the end, the existing iterators are not invalidated.
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BlocksList data;
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mutable std::mutex mutex;
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bool delay_read_for_global_subqueries = false;
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protected:
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StorageMemory(const StorageID & table_id_, ColumnsDescription columns_description_, ConstraintsDescription constraints_);
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};
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}
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