ClickHouse/dbms/include/DB/Storages/MergeTree/SimpleMergeSelector.h

#pragma once

#include <DB/Storages/MergeTree/MergeSelector.h>


namespace DB
{

class SimpleMergeSelector : public IMergeSelector
{
public:
	struct Settings
	{
		size_t min_parts_to_merge_at_once = 8;
		size_t max_parts_to_merge_at_once = 100;

		double max_nonuniformity_of_sizes_to_merge = 2;
	};

	SimpleMergeSelector(const Settings & settings) : settings(settings) {}

	PartsInPartition select(
		const Partitions & partitions,
		CanMergePart can_merge_part,
		CanMergeAdjacent can_merge_adjacent,
		const size_t max_total_size_to_merge,
		bool aggressive_mode) override
	{
		Estimator estimator;

		for (const auto & partition : partitions)
			selectWithinPartition(partition, can_merge_part, can_merge_adjacent, max_total_size_to_merge, aggressive_mode, estimator);

		return estimator.getBest();
	}

private:
	const Settings settings;

	struct Estimator
	{
		void consider(PartsInPartition && parts, size_t sum_size)
		{
			if (!min_size || sum_size < min_size)
			{
				min_size = sum_size;
				best = std::move(parts);
			}
		}

		PartsInPartition getBest()
		{
			return std::move(best);
		}

		size_t min_size = 0;
		PartsInPartition best;
	};

	void selectWithinPartition(
		const PartsInPartition & parts,
		CanMergePart can_merge_part,
		CanMergeAdjacent can_merge_adjacent,
		const size_t max_total_size_to_merge,
		bool aggressive_mode,
		Estimator & estimator)
	{
		if (parts.size() <= 1)
			return;

		for (auto left_it = parts.begin(); left_it != parts.end(); ++left_it)
		{
			if (!can_merge_part(*left_it))
				continue;

			auto right_it = left_it;
			auto prev_right_it = right_it;
			++right_it;

			size_t count = 1;
			size_t sum_size = left_it->size;
			size_t max_size = left_it->size;
			PartsInPartition candidate;
			candidate.push_back(*left_it);

			for (; right_it != parts.end(); ++right_it)
			{
				++count;
				sum_size += right_it->size;
				if (right_it->size > max_size)
					max_size = right_it->size;

				double non_uniformity = static_cast<double>(max_size * count) / sum_size;

				if (max_total_size_to_merge && sum_size > max_total_size_to_merge)
					break;

				if (count > settings.max_parts_to_merge_at_once)
					break;

				if (count < settings.min_parts_to_merge_at_once && count != parts.size())
					continue;

				if (!aggressive_mode && non_uniformity > settings.max_nonuniformity_of_sizes_to_merge)
					continue;

				if (!can_merge_part(*right_it))
					break;

				if (!can_merge_adjacent(*prev_right_it, *right_it))
					break;

				candidate.push_back(*right_it);

				prev_right_it = right_it;
			}

			estimator.consider(std::move(candidate), sum_size);
		}
	}
};

}