ClickHouse/dbms/include/DB/DataStreams/DistinctBlockInputStream.h
2016-12-23 13:01:35 +05:00

55 lines
1.3 KiB
C++

#pragma once
#include <DB/DataStreams/IProfilingBlockInputStream.h>
#include <DB/Interpreters/Limits.h>
#include <DB/Interpreters/SetVariants.h>
namespace DB
{
/** This class is intended for implementation of SELECT DISTINCT clause and
* leaves only unique rows in the stream.
*
* To optimize the SELECT DISTINCT ... LIMIT clause we can
* set limit_hint to non zero value. So we stop emitting new rows after
* count of already emitted rows will reach the limit_hint.
*/
class DistinctBlockInputStream : public IProfilingBlockInputStream
{
public:
/// Empty columns_ means all collumns.
DistinctBlockInputStream(BlockInputStreamPtr input_, const Limits & limits, size_t limit_hint_, Names columns_);
String getName() const override { return "Distinct"; }
String getID() const override;
protected:
Block readImpl() override;
private:
bool checkLimits() const;
ConstColumnPlainPtrs getKeyColumns(const Block & block) const;
template <typename Method>
void buildFilter(
Method & method,
const ConstColumnPlainPtrs & key_columns,
IColumn::Filter & filter,
size_t rows) const;
Names columns_names;
SetVariants data;
Sizes key_sizes;
size_t limit_hint;
/// Restrictions on the maximum size of the output data.
size_t max_rows;
size_t max_bytes;
OverflowMode overflow_mode;
};
}