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42 lines
1.0 KiB
C++
42 lines
1.0 KiB
C++
#pragma once
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#include <DataStreams/IProfilingBlockInputStream.h>
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#include <Common/HashTable/HashMap.h>
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#include <Common/UInt128.h>
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namespace DB
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{
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/** Implements LIMIT BY clause witch can be used to obtain a "top N by subgroup".
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*
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* For example, if you have table T like this (Num: 1 1 3 3 3 4 4 5 7 7 7 7),
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* the query SELECT Num FROM T LIMIT 2 BY Num
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* will give you the following result: (Num: 1 1 3 3 4 4 5 7 7).
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*/
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class LimitByBlockInputStream : public IProfilingBlockInputStream
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{
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public:
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LimitByBlockInputStream(const BlockInputStreamPtr & input, size_t group_size_, const Names & columns);
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String getName() const override { return "LimitBy"; }
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Block getHeader() const override { return children.at(0)->getHeader(); }
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protected:
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Block readImpl() override;
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private:
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ColumnRawPtrs getKeyColumns(Block & block) const;
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private:
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using MapHashed = HashMap<UInt128, UInt64, UInt128TrivialHash>;
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const Names columns_names;
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const size_t group_size;
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MapHashed keys_counts;
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};
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}
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