Merge branch 'partial-sort' of https://github.com/postgrespro/postgres_cluster into partial-sort

Author: knizhnik

src/backend/optimizer/path/pathkeys.c

@@ -330,6 +330,26 @@ pathkeys_contained_in(List *keys1, List *keys2)
 	return false;
 }
 
+int
+pathkeys_get_prefix(List *keys1, List *keys2)
+{
+	ListCell   *key1,
+			   *key2;
+
+	int prefix = 0;
+
+	forboth(key1, keys1, key2, keys2)
+	{
+		PathKey    *pathkey1 = (PathKey *) lfirst(key1);
+		PathKey    *pathkey2 = (PathKey *) lfirst(key2);
+
+		if (pathkey1 != pathkey2)
+			break;
+		prefix += 1;
+	}
+	return prefix;
+}
+
 /*
  * get_cheapest_path_for_pathkeys
  *	  Find the cheapest path (according to the specified criterion) that

src/backend/optimizer/plan/createplan.c

@@ -4049,6 +4049,17 @@ make_sort(PlannerInfo *root, Plan *lefttree, int numCols,
 			  0.0,
 			  work_mem,
 			  limit_tuples);
+	if (lefttree->type == T_IndexOnlyScan && root->simple_rel_array_size == 2)
+	{
+		RelOptInfo* relinfo = root->simple_rel_array[1];
+		IndexOptInfo* indexinfo = linitial(root->simple_rel_array[1]->indexlist);
+		IndexOnlyScan* indexscan = (IndexOnlyScan*)lefttree;
+		List *index_pathkeys = build_index_pathkeys(root, indexinfo, indexscan->indexorderdir);
+		int prefix_len = pathkeys_get_prefix(root->query_pathkeys, index_pathkeys);
+		sort_path.total_cost -= sort_path.startup_cost;
+		sort_path.startup_cost /= (prefix_len+1);
+		sort_path.total_cost += sort_path.startup_cost;
+	}
 	plan->startup_cost = sort_path.startup_cost;
 	plan->total_cost = sort_path.total_cost;
 	plan->targetlist = lefttree->targetlist;

src/include/optimizer/paths.h

@@ -167,6 +167,7 @@ typedef enum
 
 extern PathKeysComparison compare_pathkeys(List *keys1, List *keys2);
 extern bool pathkeys_contained_in(List *keys1, List *keys2);
+extern int  pathkeys_get_prefix(List *keys1, List *keys2);
 extern Path *get_cheapest_path_for_pathkeys(List *paths, List *pathkeys,
 							   Relids required_outer,
 							   CostSelector cost_criterion);