Fix incorrect is-this-the-topmost-join tests in parallel planning.

Two callers of generate_useful_gather_paths were testing the wrong
thing when deciding whether to call that function: they checked for
being at the top of the current join subproblem, rather than being at
the actual top join.  This'd result in failing to construct parallel
paths for a sub-join for which they might be useful.

While set_rel_pathlist() isn't actively broken, it seems best to
make its identical-in-intention test for this be like the other two.

This has been wrong all along, but given the lack of field complaints
I'm hesitant to back-patch into stable branches; we usually prefer
to avoid non-bug-fix changes in plan choices in minor releases.
It seems not too late for v15 though.

Richard Guo, reviewed by Antonin Houska and Tom Lane

Discussion: https://postgr.es/m/CAMbWs4-mH8Zf87-w+3P2J=nJB+5OyicO28ia9q_9o=Lamf_VHg@mail.gmail.com

src/backend/optimizer/geqo/geqo_eval.c

@@ -273,7 +273,7 @@ merge_clump(PlannerInfo *root, List *clumps, Clump *new_clump, int num_gene,
 				 * rel once we know the final targetlist (see
 				 * grouping_planner).
 				 */
-				if (old_clump->size + new_clump->size < num_gene)
+				if (!bms_equal(joinrel->relids, root->all_baserels))
 					generate_useful_gather_paths(root, joinrel, false);
 
 				/* Find and save the cheapest paths for this joinrel */

src/backend/optimizer/path/allpaths.c

@@ -553,12 +553,11 @@ set_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
 	 * its own pool of workers.  Instead, we'll consider gathering partial
 	 * paths for the parent appendrel.
 	 *
-	 * Also, if this is the topmost scan/join rel (that is, the only baserel),
-	 * we postpone gathering until the final scan/join targetlist is available
-	 * (see grouping_planner).
+	 * Also, if this is the topmost scan/join rel, we postpone gathering until
+	 * the final scan/join targetlist is available (see grouping_planner).
 	 */
 	if (rel->reloptkind == RELOPT_BASEREL &&
-		bms_membership(root->all_baserels) != BMS_SINGLETON)
+		!bms_equal(rel->relids, root->all_baserels))
 		generate_useful_gather_paths(root, rel, false);
 
 	/* Now find the cheapest of the paths for this rel */
@@ -3402,7 +3401,7 @@ standard_join_search(PlannerInfo *root, int levels_needed, List *initial_rels)
 			 * partial paths.  We'll do the same for the topmost scan/join rel
 			 * once we know the final targetlist (see grouping_planner).
 			 */
-			if (lev < levels_needed)
+			if (!bms_equal(rel->relids, root->all_baserels))
 				generate_useful_gather_paths(root, rel, false);
 
 			/* Find and save the cheapest paths for this rel */