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

#pragma once

#include <queue>
#include <Poco/TemporaryFile.h>

#include <common/logger_useful.h>

#include <DB/Core/SortDescription.h>

#include <DB/DataStreams/IProfilingBlockInputStream.h>
#include <DB/DataStreams/NativeBlockInputStream.h>

#include <DB/IO/ReadBufferFromFile.h>
#include <DB/IO/CompressedReadBuffer.h>


namespace DB
{

/** Merges stream of sorted each-separately blocks to sorted as-a-whole stream of blocks.
  * If data to sort is too much, could use external sorting, with temporary files.
  */

/** Part of implementation. Merging array of ready (already read from somewhere) blocks.
  * Returns result of merge as stream of blocks, not more than 'max_merged_block_size' rows in each.
  */
class MergeSortingBlocksBlockInputStream : public IProfilingBlockInputStream
{
public:
	/// limit - if not 0, allowed to return just first 'limit' rows in sorted order.
	MergeSortingBlocksBlockInputStream(Blocks & blocks_, SortDescription & description_,
		size_t max_merged_block_size_, size_t limit_ = 0);

	String getName() const override { return "MergeSortingBlocks"; }
	String getID() const override { return getName(); }

protected:
	Block readImpl() override;

private:
	Blocks & blocks;
	SortDescription description;
	size_t max_merged_block_size;
	size_t limit;
	size_t total_merged_rows = 0;

	using CursorImpls = std::vector<SortCursorImpl>;
	CursorImpls cursors;

	bool has_collation = false;

	std::priority_queue<SortCursor> queue;
	std::priority_queue<SortCursorWithCollation> queue_with_collation;

	/** Two different cursors are supported - with and without Collation.
	 *  Templates are used (instead of virtual functions in SortCursor) for zero-overhead.
	 */
	template <typename TSortCursor>
	Block mergeImpl(std::priority_queue<TSortCursor> & queue);
};


class MergeSortingBlockInputStream : public IProfilingBlockInputStream
{
public:
	/// limit - if not 0, allowed to return just first 'limit' rows in sorted order.
	MergeSortingBlockInputStream(BlockInputStreamPtr input_, SortDescription & description_,
		size_t max_merged_block_size_, size_t limit_,
		size_t max_bytes_before_external_sort_, const std::string & tmp_path_)
		: description(description_), max_merged_block_size(max_merged_block_size_), limit(limit_),
		max_bytes_before_external_sort(max_bytes_before_external_sort_), tmp_path(tmp_path_)
	{
		children.push_back(input_);
	}

	String getName() const override { return "MergeSorting"; }

	String getID() const override
	{
		std::stringstream res;
		res << "MergeSorting(" << children.back()->getID();

		for (size_t i = 0; i < description.size(); ++i)
			res << ", " << description[i].getID();

		res << ")";
		return res.str();
	}

protected:
	Block readImpl() override;

private:
	SortDescription description;
	size_t max_merged_block_size;
	size_t limit;

	size_t max_bytes_before_external_sort;
	const std::string tmp_path;

	Logger * log = &Logger::get("MergeSortingBlockInputStream");

	Blocks blocks;
	size_t sum_bytes_in_blocks = 0;
	std::unique_ptr<IBlockInputStream> impl;

	/// Before operation, will remove constant columns from blocks. And after, place constant columns back.
	/// (to avoid excessive virtual function calls and because constants cannot be serialized in Native format for temporary files)
	/// Save original block structure here.
	Block sample_block;

	/// Everything below is for external sorting.
	std::vector<std::unique_ptr<Poco::TemporaryFile>> temporary_files;

	/// For reading data from temporary file.
	struct TemporaryFileStream
	{
		ReadBufferFromFile file_in;
		CompressedReadBuffer compressed_in;
		BlockInputStreamPtr block_in;

		TemporaryFileStream(const std::string & path)
			: file_in(path), compressed_in(file_in), block_in(std::make_shared<NativeBlockInputStream>(compressed_in)) {}
	};

	std::vector<std::unique_ptr<TemporaryFileStream>> temporary_inputs;

	BlockInputStreams inputs_to_merge;
};

}