Fix wrong costing of Sort under Gather Merge.

robertmhaas · robertmhaas · commit dc02c7bca4dc · 2017-03-22T14:45:14.000-04:00
There's no mechanism for such a sort to become a top-N sort, so we should pass -1 rather than limit_tuples to cost_sort(). Rushabh Lathia, per a report from Mithun Cy Discussion: http://postgr.es/m/CAGPqQf1akRcSgC9=6iwx=sEPap9UvPpHJLzg8_N+OuHdb6fL+g@mail.gmail.com
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
@@ -4416,7 +4416,7 @@ create_ordered_paths(PlannerInfo *root,
 											 ordered_rel,
 											 cheapest_partial_path,
 											 root->sort_pathkeys,
-											 limit_tuples);
+											 -1.0);
 
 			total_groups = cheapest_partial_path->rows *
 				cheapest_partial_path->parallel_workers;
