Attempt to improve merge selecting algorithm [#METR-21840].

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

@@ -17,10 +17,13 @@ public:
 		  * This behaviour balances merge-tree workload.
 		  * It called 'base', because merge-tree depth could be estimated as logarithm with that base.
 		  */
-		double base = 2;
+		double base = 5;
 
-		time_t lower_base_after_seconds = 300;
-		size_t lower_base_after_num_parts = 20;
+		time_t lower_base_after_seconds_start = 300;
+		time_t lower_base_after_seconds_end = 3600;
+
+		size_t lower_base_after_num_parts_start = 10;
+		size_t lower_base_after_num_parts_end = 50;
 
 		/// Zero means unlimited.
 		size_t max_parts_to_merge_at_once = 100;

dbms/src/Storages/MergeTree/SimpleMergeSelector.cpp

 #include <DB/Storages/MergeTree/SimpleMergeSelector.h>
+#include <DB/Common/interpolate.h>
 
 #include <cmath>
 
@@ -82,11 +83,13 @@ void selectWithinPartition(
 	if (parts_count <= 1)
 		return;
 
-	double actual_base = settings.base;
-
-	if (parts_count > settings.lower_base_after_num_parts
-		|| current_min_part_age > settings.lower_base_after_seconds)
-		actual_base = 1;
+	double actual_base = std::max(1.0, std::min(
+		settings.base,
+		std::min(
+			interpolateLinear(settings.base, 1.0, (static_cast<double>(parts_count) - settings.lower_base_after_num_parts_start)
+				/ (settings.lower_base_after_num_parts_end - settings.lower_base_after_num_parts_start)),
+			interpolateLinear(settings.base, 1.0, (static_cast<double>(current_min_part_age) - settings.lower_base_after_seconds_start)
+				/ (settings.lower_base_after_seconds_end - settings.lower_base_after_seconds_start)))));
 
 	for (size_t begin = 0; begin < parts_count; ++begin)
 	{

dbms/src/Storages/tests/merge_selector2.cpp

@@ -21,13 +21,13 @@ int main(int argc, char ** argv)
 	IMergeSelector::Partitions partitions(1);
 	IMergeSelector::PartsInPartition & parts = partitions.back();
 
-/*	SimpleMergeSelector::Settings settings;
-	SimpleMergeSelector selector(settings);*/
+	SimpleMergeSelector::Settings settings;
+	SimpleMergeSelector selector(settings);
 
-	LevelMergeSelector::Settings settings;
+/*	LevelMergeSelector::Settings settings;
 	settings.min_parts_to_merge = 8;
 	settings.max_parts_to_merge = 16;
-	LevelMergeSelector selector(settings);
+	LevelMergeSelector selector(settings);*/
 
 	ReadBufferFromFileDescriptor in(STDIN_FILENO);