Disable run condition optimization for some WindowFuncs

david-rowley · david-rowley · commit 7e5d20bbd13f · 2024-05-01T16:35:37.000+12:00
94985c2 added code to detect when WindowFuncs were monotonic and allowed additional quals to be "pushed down" into the subquery to be used as WindowClause runConditions in order to short-circuit execution in nodeWindowAgg.c. The Node representation of runConditions wasn't well selected and because we do qual pushdown before planning the subquery, the planning of the subquery could perform subquery pull-up of nested subqueries. For WindowFuncs with args, the arguments could be changed after pushing the qual down to the subquery. This was made more difficult by the fact that the code duplicated the WindowFunc inside an OpExpr to include in the WindowClauses runCondition field. This could result in duplication of subqueries and a pull-up of such a subquery could result in another initplan parameter being issued for the 2nd version of the subplan. This could result in errors such as: ERROR: WindowFunc not found in subplan target lists Here in the backbranches, we don't have the flexibility to improve the Node representation to resolve this, so instead we just disable the runCondition optimization for ntile() unless the argument is a Const, (v16 only) and likewise for count(expr) (both v15 and v16). count(*) is unaffected. All other window functions which support this optimization all take zero arguments and therefore are unaffected. Bug: #18170 Reported-by: Zuming Jiang Discussion: https://postgr.es/m/18170-f1d17bf9a0d58b24@postgresql.org Backpatch-through 15 (master will be fixed independently)
diff --git a/src/backend/utils/adt/int8.c b/src/backend/utils/adt/int8.c
@@ -833,6 +833,21 @@ int8inc_support(PG_FUNCTION_ARGS)
 		SupportRequestWFuncMonotonic *req = (SupportRequestWFuncMonotonic *) rawreq;
 		MonotonicFunction monotonic = MONOTONICFUNC_NONE;
 		int			frameOptions = req->window_clause->frameOptions;
+		WindowFunc *wfunc = req->window_func;
+
+		if (list_length(wfunc->args) == 1)
+		{
+			Node *expr = eval_const_expressions(NULL, linitial(wfunc->args));
+
+			/*
+			 * Due to the Node representation of WindowClause runConditions in
+			 * version prior to v17, we need to insist that the count arg is
+			 * Const to allow safe application of the runCondition
+			 * optimization.
+			 */
+			if (!IsA(expr, Const))
+				PG_RETURN_POINTER(NULL);
+		}
 
 		/* No ORDER BY clause then all rows are peers */
 		if (req->window_clause->orderClause == NIL)
diff --git a/src/backend/utils/adt/windowfuncs.c b/src/backend/utils/adt/windowfuncs.c
@@ -14,6 +14,7 @@
 #include "postgres.h"
 
 #include "nodes/supportnodes.h"
+#include "optimizer/optimizer.h"
 #include "utils/builtins.h"
 #include "windowapi.h"
 
diff --git a/src/test/regress/expected/window.out b/src/test/regress/expected/window.out
@@ -3481,13 +3481,13 @@ EXPLAIN (COSTS OFF)
 SELECT * FROM
   (SELECT empno,
           salary,
-          count(empno) OVER (ORDER BY salary DESC) c
+          count(1) OVER (ORDER BY salary DESC) c
    FROM empsalary) emp
 WHERE c <= 3;
-                       QUERY PLAN                        
----------------------------------------------------------
+                QUERY PLAN                 
+-------------------------------------------
  WindowAgg
-   Run Condition: (count(empsalary.empno) OVER (?) <= 3)
+   Run Condition: (count(1) OVER (?) <= 3)
    ->  Sort
          Sort Key: empsalary.salary DESC
          ->  Seq Scan on empsalary
@@ -3496,7 +3496,7 @@ WHERE c <= 3;
 SELECT * FROM
   (SELECT empno,
           salary,
-          count(empno) OVER (ORDER BY salary DESC) c
+          count(1) OVER (ORDER BY salary DESC) c
    FROM empsalary) emp
 WHERE c <= 3;
  empno | salary | c 
@@ -3608,19 +3608,19 @@ WHERE rn < 3;
                ->  Seq Scan on empsalary
 (6 rows)
 
--- likewise with count(empno) instead of row_number()
+-- likewise with count(1) instead of row_number()
 EXPLAIN (COSTS OFF)
 SELECT * FROM
   (SELECT empno,
           depname,
           salary,
-          count(empno) OVER (PARTITION BY depname ORDER BY salary DESC) c
+          count(1) OVER (PARTITION BY depname ORDER BY salary DESC) c
    FROM empsalary) emp
 WHERE c <= 3;
                          QUERY PLAN                         
 ------------------------------------------------------------
  WindowAgg
-   Run Condition: (count(empsalary.empno) OVER (?) <= 3)
+   Run Condition: (count(1) OVER (?) <= 3)
    ->  Sort
          Sort Key: empsalary.depname, empsalary.salary DESC
          ->  Seq Scan on empsalary
@@ -3631,7 +3631,7 @@ SELECT * FROM
   (SELECT empno,
           depname,
           salary,
-          count(empno) OVER (PARTITION BY depname ORDER BY salary DESC) c
+          count(1) OVER (PARTITION BY depname ORDER BY salary DESC) c
    FROM empsalary) emp
 WHERE c <= 3;
  empno |  depname  | salary | c 
@@ -3653,21 +3653,21 @@ SELECT * FROM
   (SELECT empno,
           depname,
           salary,
-          count(empno) OVER () c
+          count(1) OVER () c
    FROM empsalary) emp
 WHERE c = 1;
-                       QUERY PLAN                       
---------------------------------------------------------
+                QUERY PLAN                
+------------------------------------------
  WindowAgg
-   Run Condition: (count(empsalary.empno) OVER (?) = 1)
+   Run Condition: (count(1) OVER (?) = 1)
    ->  Seq Scan on empsalary
 (3 rows)
 
 -- Some more complex cases with multiple window clauses
 EXPLAIN (COSTS OFF)
 SELECT * FROM
   (SELECT *,
-          count(salary) OVER (PARTITION BY depname || '') c1, -- w1
+          count(1) OVER (PARTITION BY depname || '') c1, -- w1
           row_number() OVER (PARTITION BY depname) rn, -- w2
           count(*) OVER (PARTITION BY depname) c2, -- w2
           count(*) OVER (PARTITION BY '' || depname) c3 -- w3
@@ -3678,7 +3678,7 @@ SELECT * FROM
  Subquery Scan on e
    ->  WindowAgg
          Filter: ((row_number() OVER (?)) <= 1)
-         Run Condition: (count(empsalary.salary) OVER (?) <= 3)
+         Run Condition: (count(1) OVER (?) <= 3)
          ->  Sort
                Sort Key: (((empsalary.depname)::text || ''::text))
                ->  WindowAgg
@@ -3694,7 +3694,7 @@ SELECT * FROM
 -- Ensure we correctly filter out all of the run conditions from each window
 SELECT * FROM
   (SELECT *,
-          count(salary) OVER (PARTITION BY depname || '') c1, -- w1
+          count(1) OVER (PARTITION BY depname || '') c1, -- w1
           row_number() OVER (PARTITION BY depname) rn, -- w2
           count(*) OVER (PARTITION BY depname) c2, -- w2
           count(*) OVER (PARTITION BY '' || depname) c3 -- w3
@@ -3765,6 +3765,24 @@ WHERE c = 1;
                ->  Seq Scan on empsalary
 (6 rows)
 
+-- Ensure we don't use a run condition when the WindowFunc arg contains a Var
+EXPLAIN (COSTS OFF)
+SELECT * FROM
+  (SELECT empno,
+          salary,
+          count(empno) OVER (ORDER BY empno DESC) c
+   FROM empsalary) emp
+WHERE c = 1;
+                  QUERY PLAN                  
+----------------------------------------------
+ Subquery Scan on emp
+   Filter: (emp.c = 1)
+   ->  WindowAgg
+         ->  Sort
+               Sort Key: empsalary.empno DESC
+               ->  Seq Scan on empsalary
+(6 rows)
+
 -- Ensure we don't use a run condition when the WindowFunc contains subplans
 EXPLAIN (COSTS OFF)
 SELECT * FROM
diff --git a/src/test/regress/sql/window.sql b/src/test/regress/sql/window.sql
@@ -1068,14 +1068,14 @@ EXPLAIN (COSTS OFF)
 SELECT * FROM
   (SELECT empno,
           salary,
-          count(empno) OVER (ORDER BY salary DESC) c
+          count(1) OVER (ORDER BY salary DESC) c
    FROM empsalary) emp
 WHERE c <= 3;
 
 SELECT * FROM
   (SELECT empno,
           salary,
-          count(empno) OVER (ORDER BY salary DESC) c
+          count(1) OVER (ORDER BY salary DESC) c
    FROM empsalary) emp
 WHERE c <= 3;
 
@@ -1131,13 +1131,13 @@ SELECT empno, depname FROM
    FROM empsalary) emp
 WHERE rn < 3;
 
--- likewise with count(empno) instead of row_number()
+-- likewise with count(1) instead of row_number()
 EXPLAIN (COSTS OFF)
 SELECT * FROM
   (SELECT empno,
           depname,
           salary,
-          count(empno) OVER (PARTITION BY depname ORDER BY salary DESC) c
+          count(1) OVER (PARTITION BY depname ORDER BY salary DESC) c
    FROM empsalary) emp
 WHERE c <= 3;
 
@@ -1146,7 +1146,7 @@ SELECT * FROM
   (SELECT empno,
           depname,
           salary,
-          count(empno) OVER (PARTITION BY depname ORDER BY salary DESC) c
+          count(1) OVER (PARTITION BY depname ORDER BY salary DESC) c
    FROM empsalary) emp
 WHERE c <= 3;
 
@@ -1157,15 +1157,15 @@ SELECT * FROM
   (SELECT empno,
           depname,
           salary,
-          count(empno) OVER () c
+          count(1) OVER () c
    FROM empsalary) emp
 WHERE c = 1;
 
 -- Some more complex cases with multiple window clauses
 EXPLAIN (COSTS OFF)
 SELECT * FROM
   (SELECT *,
-          count(salary) OVER (PARTITION BY depname || '') c1, -- w1
+          count(1) OVER (PARTITION BY depname || '') c1, -- w1
           row_number() OVER (PARTITION BY depname) rn, -- w2
           count(*) OVER (PARTITION BY depname) c2, -- w2
           count(*) OVER (PARTITION BY '' || depname) c3 -- w3
@@ -1175,7 +1175,7 @@ SELECT * FROM
 -- Ensure we correctly filter out all of the run conditions from each window
 SELECT * FROM
   (SELECT *,
-          count(salary) OVER (PARTITION BY depname || '') c1, -- w1
+          count(1) OVER (PARTITION BY depname || '') c1, -- w1
           row_number() OVER (PARTITION BY depname) rn, -- w2
           count(*) OVER (PARTITION BY depname) c2, -- w2
           count(*) OVER (PARTITION BY '' || depname) c3 -- w3
@@ -1215,6 +1215,15 @@ SELECT * FROM
    FROM empsalary) emp
 WHERE c = 1;
 
+-- Ensure we don't use a run condition when the WindowFunc arg contains a Var
+EXPLAIN (COSTS OFF)
+SELECT * FROM
+  (SELECT empno,
+          salary,
+          count(empno) OVER (ORDER BY empno DESC) c
+   FROM empsalary) emp
+WHERE c = 1;
+
 -- Ensure we don't use a run condition when the WindowFunc contains subplans
 EXPLAIN (COSTS OFF)
 SELECT * FROM
