ClickHouse/dbms/include/DB/DataStreams/DistinctBlockInputStream.h

#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,
		SetVariants & variants) 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;
};

}