Fix force_parallel_mode to work as intended.

The force_parallel_mode GUC is meant to facilitate testing of parallel
query plans, but it currently allows non-parallel plans to win even when
force_parallel_mode is enabled.
This behavior makes it difficult to debug parallel plans as intended.

Correct the behavior for testing purposes.

Authored-by: Zhang Mingli [email protected]

Author: avamingli

src/backend/optimizer/plan/planner.c

@@ -593,6 +593,8 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
 	{
 		Path	   *cheapest_partial_path;
 		cheapest_partial_path = linitial(final_rel->partial_pathlist);
+		if (force_parallel_mode)
+			final_rel->pathlist = NIL;
 		add_path(final_rel, cheapest_partial_path, root);
 		set_cheapest(final_rel);
 	}