Remove some bogus logic from create_gather_merge_plan.

This logic was adapated from create_merge_append_plan, but the two
cases aren't really analogous, because create_merge_append_plan is not
projection-capable and must therefore have a tlist identical to that
of the underlying paths.  Overwriting the tlist of Gather Merge with
whatever the underlying plan happens to produce is no good at all.

Patch by me, reviewed by Rushabh Lathia, who also reported the issue
and made an initial attempt at a fix.

Discussion: http://postgr.es/m/CA+Tgmob_-oHEOBfT9S25bjqokdqv8e8xEmh9zOY+3MPr_LmuhA@mail.gmail.com

src/backend/optimizer/plan/createplan.c

@@ -1469,17 +1469,12 @@ create_gather_merge_plan(PlannerInfo *root, GatherMergePath *best_path)
 	GatherMerge *gm_plan;
 	Plan	   *subplan;
 	List	   *pathkeys = best_path->path.pathkeys;
-	int			numsortkeys;
-	AttrNumber *sortColIdx;
-	Oid		   *sortOperators;
-	Oid		   *collations;
-	bool	   *nullsFirst;
 	List	   *tlist = build_path_tlist(root, &best_path->path);
 
 	/* As with Gather, it's best to project away columns in the workers. */
 	subplan = create_plan_recurse(root, best_path->subpath, CP_EXACT_TLIST);
 
-	/* See create_merge_append_plan for why there's no make_xxx function */
+	/* Create a shell for a GatherMerge plan. */
 	gm_plan = makeNode(GatherMerge);
 	gm_plan->plan.targetlist = tlist;
 	gm_plan->num_workers = best_path->num_workers;
@@ -1488,46 +1483,25 @@ create_gather_merge_plan(PlannerInfo *root, GatherMergePath *best_path)
 	/* Gather Merge is pointless with no pathkeys; use Gather instead. */
 	Assert(pathkeys != NIL);
 
-	/* Compute sort column info, and adjust GatherMerge tlist as needed */
-	(void) prepare_sort_from_pathkeys(&gm_plan->plan, pathkeys,
-									  best_path->path.parent->relids,
-									  NULL,
-									  true,
-									  &gm_plan->numCols,
-									  &gm_plan->sortColIdx,
-									  &gm_plan->sortOperators,
-									  &gm_plan->collations,
-									  &gm_plan->nullsFirst);
-
-
 	/* Compute sort column info, and adjust subplan's tlist as needed */
 	subplan = prepare_sort_from_pathkeys(subplan, pathkeys,
 										 best_path->subpath->parent->relids,
 										 gm_plan->sortColIdx,
 										 false,
-										 &numsortkeys,
-										 &sortColIdx,
-										 &sortOperators,
-										 &collations,
-										 &nullsFirst);
+										 &gm_plan->numCols,
+										 &gm_plan->sortColIdx,
+										 &gm_plan->sortOperators,
+										 &gm_plan->collations,
+										 &gm_plan->nullsFirst);
 
-	/* As for MergeAppend, check that we got the same sort key information. */
-	Assert(numsortkeys == gm_plan->numCols);
-	if (memcmp(sortColIdx, gm_plan->sortColIdx,
-			   numsortkeys * sizeof(AttrNumber)) != 0)
-		elog(ERROR, "GatherMerge child's targetlist doesn't match GatherMerge");
-	Assert(memcmp(sortOperators, gm_plan->sortOperators,
-				  numsortkeys * sizeof(Oid)) == 0);
-	Assert(memcmp(collations, gm_plan->collations,
-				  numsortkeys * sizeof(Oid)) == 0);
-	Assert(memcmp(nullsFirst, gm_plan->nullsFirst,
-				  numsortkeys * sizeof(bool)) == 0);
 
 	/* Now, insert a Sort node if subplan isn't sufficiently ordered */
 	if (!pathkeys_contained_in(pathkeys, best_path->subpath->pathkeys))
-		subplan = (Plan *) make_sort(subplan, numsortkeys,
-									 sortColIdx, sortOperators,
-									 collations, nullsFirst);
+		subplan = (Plan *) make_sort(subplan, gm_plan->numCols,
+									 gm_plan->sortColIdx,
+									 gm_plan->sortOperators,
+									 gm_plan->collations,
+									 gm_plan->nullsFirst);
 
 	/* Now insert the subplan under GatherMerge. */
 	gm_plan->plan.lefttree = subplan;