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58 lines
2.0 KiB
C++
58 lines
2.0 KiB
C++
#pragma once
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#include <base/shared_ptr_helper.h>
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#include <optional>
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#include <Storages/IStorage.h>
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namespace DB
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{
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class Context;
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/** Implements a table engine for the system table "numbers".
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* The table contains the only column number UInt64.
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* From this table, you can read all natural numbers, starting from 0 (to 2^64 - 1, and then again).
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*
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* You could also specify a limit (how many numbers to give).
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* If multithreaded is specified, numbers will be generated in several streams
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* (and result could be out of order). If both multithreaded and limit are specified,
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* the table could give you not exactly 1..limit range, but some arbitrary 'limit' numbers.
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*
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* In multithreaded case, if even_distributed is False, implementation with atomic is used,
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* and result is always in [0 ... limit - 1] range.
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*/
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class StorageSystemNumbers final : public shared_ptr_helper<StorageSystemNumbers>, public IStorage
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{
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friend struct shared_ptr_helper<StorageSystemNumbers>;
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public:
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std::string getName() const override { return "SystemNumbers"; }
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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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SelectQueryInfo & query_info,
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ContextPtr 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 hasEvenlyDistributedRead() const override { return true; }
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bool isSystemStorage() const override { return true; }
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bool supportsTransactions() const override { return true; }
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private:
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bool multithreaded;
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bool even_distribution;
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std::optional<UInt64> limit;
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UInt64 offset;
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protected:
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/// If even_distribution is true, numbers are distributed evenly between streams.
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/// Otherwise, streams concurrently increment atomic.
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StorageSystemNumbers(const StorageID & table_id, bool multithreaded_, std::optional<UInt64> limit_ = std::nullopt, UInt64 offset_ = 0, bool even_distribution_ = true);
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};
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}
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