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
	{
		/// Zero means unlimited.
		size_t max_parts_to_merge_at_once = 100;

		/** Minimum ratio of size of one part to all parts in set of parts to merge (for usual cases).
		  * For example, if all parts have equal size, it means, that at least 'base' number of parts should be merged.
		  * If parts has non-uniform sizes, then minumum number of parts to merge is effectively increased.
		  * This behaviour balances merge-tree workload.
		  * It called 'base', because merge-tree depth could be estimated as logarithm with that base.
		  *
		  * If base is higher - then tree gets more wide and narrow, lowering write amplification.
		  * If base is lower - then merges occurs more frequently, lowering number of parts in average.
		  *
		  * We need some balance between write amplification and number of parts.
		  */
		double base = 5;

		/** Base is lowered until 1 (effectively means "merge any two parts") depending on several variables:
		  *
		  * 1. Total number of parts in partition. If too much - then base is lowered.
		  * It means: when too much parts - do merges more urgently.
		  *
		  * 2. Minimum age of parts participating in merge. If higher age - then base is lowered.
		  * It means: do less wide merges only rarely.
		  *
		  * 3. Sum size of parts participating in merge. If higher - then more age is required to lower base. So, base is lowered slower.
		  * It means: for small parts, it's worth to merge faster, even not so wide or balanced.
		  *
		  * We have multivariative dependency. Let it be logarithmic of size and somewhat multi-linear by other variables,
		  *  between some boundary points, and constant outside.
		  */

		size_t min_size_to_lower_base = 1024 * 1024;
		size_t max_size_to_lower_base = 100ULL * 1024 * 1024 * 1024;

		time_t min_age_to_lower_base_at_min_size = 10;
		time_t min_age_to_lower_base_at_max_size = 10;
		time_t max_age_to_lower_base_at_min_size = 3600;
		time_t max_age_to_lower_base_at_max_size = 30 * 86400;

		size_t min_parts_to_lower_base = 10;
		size_t max_parts_to_lower_base = 50;

		/// Add this to size before all calculations. It means: merging even very small parts has it's fixed cost.
		size_t size_fixed_cost_to_add = 5 * 1024 * 1024;

		/** Heuristic:
		  * Make some preference for ranges, that sum_size is like (in terms of ratio) to part previous at left.
		  */
		bool enable_heuristic_to_align_parts = true;
		double heuristic_to_align_parts_min_ratio_of_sum_size_to_prev_part = 0.9;
		double heuristic_to_align_parts_max_absolute_difference_in_powers_of_two = 0.5;
		double heuristic_to_align_parts_max_score_adjustment = 0.75;

		/** Heuristic:
		  * From right side of range, remove all parts, that size is less than specified ratio of sum_size.
		  */
		bool enable_heuristic_to_remove_small_parts_at_right = true;
		double heuristic_to_remove_small_parts_at_right_max_ratio = 0.01;
	};

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

	PartsInPartition select(
		const Partitions & partitions,
		const size_t max_total_size_to_merge) override;

private:
	const Settings settings;
};

}