Trim ORDER BY/DISTINCT aggregate pathkeys in gather_grouping_paths

Similar to d8a295389, trim off any PathKeys which are for ORDER BY /
DISTINCT aggregate functions from the PathKey List for the Gather Merge
paths created by gather_grouping_paths().  These additional PathKeys are
not valid to use after grouping has taken place as these PathKeys belong
to columns which are inputs to an aggregate function and, therefore are
unavailable after aggregation.

Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/cf63174c-8c89-3953-cb49-48f41f74941a@gmail.com
Backpatch-through: 16, where 1349d2790 was added

src/backend/optimizer/plan/planner.c

@@ -7349,13 +7349,24 @@ gather_grouping_paths(PlannerInfo *root, RelOptInfo *rel)
 {
 	ListCell   *lc;
 	Path	   *cheapest_partial_path;
+	List	   *groupby_pathkeys;
+
+	/*
+	 * This occurs after any partial aggregation has taken place, so trim off
+	 * any pathkeys added for ORDER BY / DISTINCT aggregates.
+	 */
+	if (list_length(root->group_pathkeys) > root->num_groupby_pathkeys)
+		groupby_pathkeys = list_copy_head(root->group_pathkeys,
+										  root->num_groupby_pathkeys);
+	else
+		groupby_pathkeys = root->group_pathkeys;
 
 	/* Try Gather for unordered paths and Gather Merge for ordered ones. */
 	generate_useful_gather_paths(root, rel, true);
 
 	/* Try cheapest partial path + explicit Sort + Gather Merge. */
 	cheapest_partial_path = linitial(rel->partial_pathlist);
-	if (!pathkeys_contained_in(root->group_pathkeys,
+	if (!pathkeys_contained_in(groupby_pathkeys,
 							   cheapest_partial_path->pathkeys))
 	{
 		Path	   *path;
@@ -7364,14 +7375,14 @@ gather_grouping_paths(PlannerInfo *root, RelOptInfo *rel)
 		total_groups =
 			cheapest_partial_path->rows * cheapest_partial_path->parallel_workers;
 		path = (Path *) create_sort_path(root, rel, cheapest_partial_path,
-										 root->group_pathkeys,
+										 groupby_pathkeys,
 										 -1.0);
 		path = (Path *)
 			create_gather_merge_path(root,
 									 rel,
 									 path,
 									 rel->reltarget,
-									 root->group_pathkeys,
+									 groupby_pathkeys,
 									 NULL,
 									 &total_groups);
 
@@ -7382,10 +7393,10 @@ gather_grouping_paths(PlannerInfo *root, RelOptInfo *rel)
 	 * Consider incremental sort on all partial paths, if enabled.
 	 *
 	 * We can also skip the entire loop when we only have a single-item
-	 * group_pathkeys because then we can't possibly have a presorted prefix
+	 * groupby_pathkeys because then we can't possibly have a presorted prefix
 	 * of the list without having the list be fully sorted.
 	 */
-	if (!enable_incremental_sort || list_length(root->group_pathkeys) == 1)
+	if (!enable_incremental_sort || list_length(groupby_pathkeys) == 1)
 		return;
 
 	/* also consider incremental sort on partial paths, if enabled */
@@ -7396,7 +7407,7 @@ gather_grouping_paths(PlannerInfo *root, RelOptInfo *rel)
 		int			presorted_keys;
 		double		total_groups;
 
-		is_sorted = pathkeys_count_contained_in(root->group_pathkeys,
+		is_sorted = pathkeys_count_contained_in(groupby_pathkeys,
 												path->pathkeys,
 												&presorted_keys);
 
@@ -7409,7 +7420,7 @@ gather_grouping_paths(PlannerInfo *root, RelOptInfo *rel)
 		path = (Path *) create_incremental_sort_path(root,
 													 rel,
 													 path,
-													 root->group_pathkeys,
+													 groupby_pathkeys,
 													 presorted_keys,
 													 -1.0);
 
@@ -7418,7 +7429,7 @@ gather_grouping_paths(PlannerInfo *root, RelOptInfo *rel)
 									 rel,
 									 path,
 									 rel->reltarget,
-									 root->group_pathkeys,
+									 groupby_pathkeys,
 									 NULL,
 									 &total_groups);