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

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#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;
2016-10-27 23:28:35 +00:00
/** 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;
};
}