Fix create_scan_plan's handling of sortgrouprefs for physical tlists.

We should only run apply_pathtarget_labeling_to_tlist if CP_LABEL_TLIST
was specified, because only in that case has use_physical_tlist checked
that the labeling will succeed; otherwise we may get an "ORDER/GROUP BY
expression not found in targetlist" error.  (This subsumes the previous
test about gating_clauses, because we reset "flags" to zero earlier
if there are gating clauses to apply.)

The only known case in which a failure can occur is with a ProjectSet
path directly atop a table scan path, although it seems likely that there
are other cases or will be such in future.  This means that the failure
is currently only visible in the v10 branch: 9.6 didn't have ProjectSet,
while in v11 and HEAD, apply_scanjoin_target_to_paths for some weird
reason is using create_projection_path not apply_projection_to_path,
masking the problem because there's a ProjectionPath in between.

Nonetheless this code is clearly wrong on its own terms, so back-patch
to 9.6 where this logic was introduced.

Per report from Regina Obe.

Discussion: https://postgr.es/m/001501d40f88$75186950$5f493bf0$@pcorp.us

src/backend/optimizer/plan/createplan.c

@@ -579,10 +579,10 @@ create_scan_plan(PlannerInfo *root, Path *best_path, int flags)
 			tlist = copyObject(((IndexPath *) best_path)->indexinfo->indextlist);
 
 			/*
-			 * Transfer any sortgroupref data to the replacement tlist, unless
-			 * we don't care because the gating Result will handle it.
+			 * Transfer sortgroupref data to the replacement tlist, if
+			 * requested (use_physical_tlist checked that this will work).
 			 */
-			if (!gating_clauses)
+			if (flags & CP_LABEL_TLIST)
 				apply_pathtarget_labeling_to_tlist(tlist, best_path->pathtarget);
 		}
 		else
@@ -596,7 +596,7 @@ create_scan_plan(PlannerInfo *root, Path *best_path, int flags)
 			else
 			{
 				/* As above, transfer sortgroupref data to replacement tlist */
-				if (!gating_clauses)
+				if (flags & CP_LABEL_TLIST)
 					apply_pathtarget_labeling_to_tlist(tlist, best_path->pathtarget);
 			}
 		}
@@ -1639,7 +1639,7 @@ create_projection_plan(PlannerInfo *root, ProjectionPath *best_path, int flags)
 		 */
 		subplan = create_plan_recurse(root, best_path->subpath, 0);
 		tlist = subplan->targetlist;
-		if ((flags & CP_LABEL_TLIST) != 0)
+		if (flags & CP_LABEL_TLIST)
 			apply_pathtarget_labeling_to_tlist(tlist,
 											   best_path->path.pathtarget);
 	}

src/test/regress/expected/tsrf.out

@@ -421,6 +421,28 @@ SELECT dataa, datab b, generate_series(1,2) g, count(*) FROM few GROUP BY CUBE(d
 (24 rows)
 
 reset enable_hashagg;
+-- case with degenerate ORDER BY
+explain (verbose, costs off)
+select 'foo' as f, generate_series(1,2) as g from few order by 1;
+                  QUERY PLAN                  
+----------------------------------------------
+ ProjectSet
+   Output: 'foo'::text, generate_series(1, 2)
+   ->  Seq Scan on public.few
+         Output: id, dataa, datab
+(4 rows)
+
+select 'foo' as f, generate_series(1,2) as g from few order by 1;
+  f  | g 
+-----+---
+ foo | 1
+ foo | 2
+ foo | 1
+ foo | 2
+ foo | 1
+ foo | 2
+(6 rows)
+
 -- data modification
 CREATE TABLE fewmore AS SELECT generate_series(1,3) AS data;
 INSERT INTO fewmore VALUES(generate_series(4,5));

src/test/regress/sql/tsrf.sql

@@ -88,6 +88,11 @@ SELECT dataa, datab b, generate_series(1,2) g, count(*) FROM few GROUP BY CUBE(d
 SELECT dataa, datab b, generate_series(1,2) g, count(*) FROM few GROUP BY CUBE(dataa, datab, g) ORDER BY g;
 reset enable_hashagg;
 
+-- case with degenerate ORDER BY
+explain (verbose, costs off)
+select 'foo' as f, generate_series(1,2) as g from few order by 1;
+select 'foo' as f, generate_series(1,2) as g from few order by 1;
+
 -- data modification
 CREATE TABLE fewmore AS SELECT generate_series(1,3) AS data;
 INSERT INTO fewmore VALUES(generate_series(4,5));