Make truncate_useless_pathkeys() consider WindowFuncs

truncate_useless_pathkeys() seems to have neglected to account for
PathKeys that might be useful for WindowClause evaluation.  Modify it so
that it properly accounts for that.

Making this work required adjusting two things:

1. Change from checking query_pathkeys to check sort_pathkeys instead.
2. Add explicit check for window_pathkeys

For #1, query_pathkeys gets set in standard_qp_callback() according to the
sort order requirements for the first operation to be applied after the
join planner is finished, so this changes depending on which upper
planner operations a particular query needs.  If the query has window
functions and no GROUP BY, then query_pathkeys gets set to
window_pathkeys.  Before this change, this meant PathKeys useful for the
ORDER BY were not accounted for in queries with window functions.

Because of #1, #2 is now required so that we explicitly check to ensure
we don't truncate away PathKeys useful for window functions.

Author: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/CAApHDvrj3HTKmXoLMbUjTO=_MNMxM=cnuCSyBKidAVibmYPnrg@mail.gmail.com

src/backend/optimizer/path/pathkeys.c

@@ -2154,14 +2154,31 @@ right_merge_direction(PlannerInfo *root, PathKey *pathkey)
  * Because we the have the possibility of incremental sort, a prefix list of
  * keys is potentially useful for improving the performance of the requested
  * ordering. Thus we return 0, if no valuable keys are found, or the number
- * of leading keys shared by the list and the requested ordering..
+ * of leading keys shared by the list and the requested ordering.
  */
 static int
 pathkeys_useful_for_ordering(PlannerInfo *root, List *pathkeys)
 {
 	int			n_common_pathkeys;
 
-	(void) pathkeys_count_contained_in(root->query_pathkeys, pathkeys,
+	(void) pathkeys_count_contained_in(root->sort_pathkeys, pathkeys,
+									   &n_common_pathkeys);
+
+	return n_common_pathkeys;
+}
+
+/*
+ * pathkeys_useful_for_windowing
+ *		Count the number of pathkeys that are useful for meeting the
+ *		query's desired sort order for window function evaluation.
+ */
+static int
+pathkeys_useful_for_windowing(PlannerInfo *root, List *pathkeys)
+{
+	int			n_common_pathkeys;
+
+	(void) pathkeys_count_contained_in(root->window_pathkeys,
+									   pathkeys,
 									   &n_common_pathkeys);
 
 	return n_common_pathkeys;
@@ -2276,6 +2293,9 @@ truncate_useless_pathkeys(PlannerInfo *root,
 
 	nuseful = pathkeys_useful_for_merging(root, rel, pathkeys);
 	nuseful2 = pathkeys_useful_for_ordering(root, pathkeys);
+	if (nuseful2 > nuseful)
+		nuseful = nuseful2;
+	nuseful2 = pathkeys_useful_for_windowing(root, pathkeys);
 	if (nuseful2 > nuseful)
 		nuseful = nuseful2;
 	nuseful2 = pathkeys_useful_for_grouping(root, pathkeys);

src/test/regress/expected/window.out

@@ -4537,6 +4537,22 @@ WHERE first_emp = 1 OR last_emp = 1;
  sales     |     4 |   4800 | 08-08-2007  |         3 |        1
 (6 rows)
 
+CREATE INDEX empsalary_salary_empno_idx ON empsalary (salary, empno);
+SET enable_seqscan = 0;
+-- Ensure no sorting is done and that the IndexScan maintains all pathkeys
+-- useful for the final sort order.
+EXPLAIN (COSTS OFF)
+SELECT salary, empno, row_number() OVER (ORDER BY salary) rn
+FROM empsalary
+ORDER BY salary, empno;
+                             QUERY PLAN                              
+---------------------------------------------------------------------
+ WindowAgg
+   Window: w1 AS (ORDER BY salary ROWS UNBOUNDED PRECEDING)
+   ->  Index Only Scan using empsalary_salary_empno_idx on empsalary
+(3 rows)
+
+RESET enable_seqscan;
 -- cleanup
 DROP TABLE empsalary;
 -- test user-defined window function with named args and default args

src/test/regress/sql/window.sql

@@ -1522,6 +1522,19 @@ SELECT * FROM
    FROM empsalary) emp
 WHERE first_emp = 1 OR last_emp = 1;
 
+CREATE INDEX empsalary_salary_empno_idx ON empsalary (salary, empno);
+
+SET enable_seqscan = 0;
+
+-- Ensure no sorting is done and that the IndexScan maintains all pathkeys
+-- useful for the final sort order.
+EXPLAIN (COSTS OFF)
+SELECT salary, empno, row_number() OVER (ORDER BY salary) rn
+FROM empsalary
+ORDER BY salary, empno;
+
+RESET enable_seqscan;
+
 -- cleanup
 DROP TABLE empsalary;