Strip off ORDER BY/DISTINCT aggregate pathkeys in create_agg_path

1349d2790 added code to adjust the PlannerInfo.group_pathkeys so that
ORDER BY / DISTINCT aggregate functions could obtain pre-sorted inputs
to allow faster execution.  That commit forgot to adjust the pathkeys in
create_agg_path().  Some code in there assumed that it was always fine
to make the AggPath's pathkeys the same as its subpath's.  That seems to
have been ok up until 1349d2790, but since that commit adds pathkeys for
columns which are within the aggregate function, those columns won't be
available above the aggregate node.  This can result in "could not find
pathkey item to sort" during create_plan().

The fix here is to strip off the additional pathkeys added by
adjust_group_pathkeys_for_groupagg().  It seems that the pathkeys here
will only ever be group_pathkeys, so all we need to do is check if the
length of the pathkey list is longer than the num_groupby_pathkeys and
get rid of the additional ones only if we see extras.

Reported-by: Justin Pryzby
Reviewed-by: Richard Guo
Discussion: https://postgr.es/m/ZQhYYRhUxpW3PSf9%40telsasoft.com
Backpatch-through: 16, where 1349d2790 was introduced

src/backend/optimizer/util/pathnode.c

@@ -3133,10 +3133,26 @@ create_agg_path(PlannerInfo *root,
 	pathnode->path.parallel_safe = rel->consider_parallel &&
 		subpath->parallel_safe;
 	pathnode->path.parallel_workers = subpath->parallel_workers;
+
 	if (aggstrategy == AGG_SORTED)
-		pathnode->path.pathkeys = subpath->pathkeys;	/* preserves order */
+	{
+		/*
+		 * Attempt to preserve the order of the subpath.  Additional pathkeys
+		 * may have been added in adjust_group_pathkeys_for_groupagg() to
+		 * support ORDER BY / DISTINCT aggregates.  Pathkeys added there
+		 * belong to columns within the aggregate function, so we must strip
+		 * these additional pathkeys off as those columns are unavailable
+		 * above the aggregate node.
+		 */
+		if (list_length(subpath->pathkeys) > root->num_groupby_pathkeys)
+			pathnode->path.pathkeys = list_copy_head(subpath->pathkeys,
+													 root->num_groupby_pathkeys);
+		else
+			pathnode->path.pathkeys = subpath->pathkeys;	/* preserves order */
+	}
 	else
 		pathnode->path.pathkeys = NIL;	/* output is unordered */
+
 	pathnode->subpath = subpath;
 
 	pathnode->aggstrategy = aggstrategy;