Allow incremental sorts for windowing functions

This expands on the work done in d2d8a229 and allows incremental sort
to be considered during create_window_paths().

Author: David Rowley
Reviewed-by: Daniel Gustafsson, Tomas Vondra
Discussion: https://postgr.es/m/CAApHDvoOHobiA2x13NtWnWLcTXYj9ddpCkv9PnAJQBMegYf_xw%40mail.gmail.com

src/backend/optimizer/plan/planner.c

@@ -4582,14 +4582,17 @@ create_window_paths(PlannerInfo *root,
 	/*
 	 * Consider computing window functions starting from the existing
 	 * cheapest-total path (which will likely require a sort) as well as any
-	 * existing paths that satisfy root->window_pathkeys (which won't).
+	 * existing paths that satisfy or partially satisfy root->window_pathkeys.
 	 */
 	foreach(lc, input_rel->pathlist)
 	{
 		Path	   *path = (Path *) lfirst(lc);
+		int			presorted_keys;
 
 		if (path == input_rel->cheapest_total_path ||
-			pathkeys_contained_in(root->window_pathkeys, path->pathkeys))
+			pathkeys_count_contained_in(root->window_pathkeys, path->pathkeys,
+										&presorted_keys) ||
+			presorted_keys > 0)
 			create_one_window_path(root,
 								   window_rel,
 								   path,
@@ -4664,18 +4667,42 @@ create_one_window_path(PlannerInfo *root,
 	{
 		WindowClause *wc = lfirst_node(WindowClause, l);
 		List	   *window_pathkeys;
+		int			presorted_keys;
+		bool		is_sorted;
 
 		window_pathkeys = make_pathkeys_for_window(root,
 												   wc,
 												   root->processed_tlist);
 
+		is_sorted = pathkeys_count_contained_in(window_pathkeys,
+												path->pathkeys,
+												&presorted_keys);
+
 		/* Sort if necessary */
-		if (!pathkeys_contained_in(window_pathkeys, path->pathkeys))
+		if (!is_sorted)
 		{
+			/*
+			 * No presorted keys or incremental sort disabled, just perform a
+			 * complete sort.
+			 */
+			if (presorted_keys == 0 || !enable_incremental_sort)
 				path = (Path *) create_sort_path(root, window_rel,
 												 path,
 												 window_pathkeys,
 												 -1.0);
+			else
+			{
+				/*
+				 * Since we have presorted keys and incremental sort is
+				 * enabled, just use incremental sort.
+				 */
+				path = (Path *) create_incremental_sort_path(root,
+															 window_rel,
+															 path,
+															 window_pathkeys,
+															 presorted_keys,
+															 -1.0);
+			}
 		}
 
 		if (lnext(activeWindows, l))

src/test/regress/expected/window.out

@@ -3200,6 +3200,50 @@ FROM empsalary;
                ->  Seq Scan on empsalary
 (5 rows)
 
+-- Test incremental sorting
+EXPLAIN (COSTS OFF)
+SELECT * FROM
+  (SELECT depname,
+          empno,
+          salary,
+          enroll_date,
+          row_number() OVER (PARTITION BY depname ORDER BY enroll_date) AS first_emp,
+          row_number() OVER (PARTITION BY depname ORDER BY enroll_date DESC) AS last_emp
+   FROM empsalary) emp
+WHERE first_emp = 1 OR last_emp = 1;
+                                    QUERY PLAN                                     
+-----------------------------------------------------------------------------------
+ Subquery Scan on emp
+   Filter: ((emp.first_emp = 1) OR (emp.last_emp = 1))
+   ->  WindowAgg
+         ->  Incremental Sort
+               Sort Key: empsalary.depname, empsalary.enroll_date
+               Presorted Key: empsalary.depname
+               ->  WindowAgg
+                     ->  Sort
+                           Sort Key: empsalary.depname, empsalary.enroll_date DESC
+                           ->  Seq Scan on empsalary
+(10 rows)
+
+SELECT * FROM
+  (SELECT depname,
+          empno,
+          salary,
+          enroll_date,
+          row_number() OVER (PARTITION BY depname ORDER BY enroll_date) AS first_emp,
+          row_number() OVER (PARTITION BY depname ORDER BY enroll_date DESC) AS last_emp
+   FROM empsalary) emp
+WHERE first_emp = 1 OR last_emp = 1;
+  depname  | empno | salary | enroll_date | first_emp | last_emp 
+-----------+-------+--------+-------------+-----------+----------
+ develop   |     8 |   6000 | 10-01-2006  |         1 |        5
+ develop   |     7 |   4200 | 01-01-2008  |         5 |        1
+ personnel |     2 |   3900 | 12-23-2006  |         1 |        2
+ personnel |     5 |   3500 | 12-10-2007  |         2 |        1
+ sales     |     1 |   5000 | 10-01-2006  |         1 |        3
+ sales     |     4 |   4800 | 08-08-2007  |         3 |        1
+(6 rows)
+
 -- cleanup
 DROP TABLE empsalary;
 -- test user-defined window function with named args and default args

src/test/regress/sql/window.sql

@@ -936,6 +936,28 @@ SELECT
   lag(1) OVER (PARTITION BY depname ORDER BY salary,enroll_date,empno)
 FROM empsalary;
 
+-- Test incremental sorting
+EXPLAIN (COSTS OFF)
+SELECT * FROM
+  (SELECT depname,
+          empno,
+          salary,
+          enroll_date,
+          row_number() OVER (PARTITION BY depname ORDER BY enroll_date) AS first_emp,
+          row_number() OVER (PARTITION BY depname ORDER BY enroll_date DESC) AS last_emp
+   FROM empsalary) emp
+WHERE first_emp = 1 OR last_emp = 1;
+
+SELECT * FROM
+  (SELECT depname,
+          empno,
+          salary,
+          enroll_date,
+          row_number() OVER (PARTITION BY depname ORDER BY enroll_date) AS first_emp,
+          row_number() OVER (PARTITION BY depname ORDER BY enroll_date DESC) AS last_emp
+   FROM empsalary) emp
+WHERE first_emp = 1 OR last_emp = 1;
+
 -- cleanup
 DROP TABLE empsalary;