[CF 5719] v9 - MergeAppend could consider sorting cheapest child path

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https://commitfest.postgresql.org/patch/5719

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Patch(es): https://www.postgresql.org/message-id/[email protected]
Author(s): Andrei Lepikhov, Alexander Pyhalov

Author: Commitfest Bot

src/backend/optimizer/path/costsize.c

@@ -512,7 +512,7 @@ cost_gather_merge(GatherMergePath *path, PlannerInfo *root,
 	logN = LOG2(N);
 
 	/* Assumed cost per tuple comparison */
-	comparison_cost = 2.0 * cpu_operator_cost;
+	comparison_cost = 3.9 * cpu_operator_cost;
 
 	/* Heap creation cost */
 	startup_cost += comparison_cost * N * logN;
@@ -2474,7 +2474,7 @@ cost_merge_append(Path *path, PlannerInfo *root,
 	logN = LOG2(N);
 
 	/* Assumed cost per tuple comparison */
-	comparison_cost = 2.0 * cpu_operator_cost;
+	comparison_cost = 3.9 * cpu_operator_cost;
 
 	/* Heap creation cost */
 	startup_cost += comparison_cost * N * logN;