Remove deduplication logic from find_window_functions

david-rowley · david-rowley · commit 7027dd499dd6 · 2026-01-26T23:27:15.000+13:00
This code thought it was optimizing WindowAgg evaluation by getting rid of duplicate WindowFuncs, but it turns out all it does today is lead to cost-underestimations and makes it possible that optimize_window_clauses could miss some of the WindowFuncs that must receive an updated winref. The deduplication likely was useful when it was first added, but since the projection code was changed in b8d7f05, the list of WindowFuncs gathered by find_window_functions isn't used during execution. Instead, the expression evaluation code will process the node's targetlist to find the WindowFuncs. The reason the deduplication could cause issues for optimize_window_clauses() is because if a WindowFunc is moved to another WindowClause, the winref is adjusted to reference the new WindowClause. If any duplicate WindowFuncs were discarded in find_window_functions() then the WindowFuncLists may not include all the WindowFuncs that need their winref adjusted. This could lead to an error message such as: ERROR: WindowFunc with winref 2 assigned to WindowAgg with winref 1 The back-branches will receive a different fix so that the WindowAgg costs are not affected. Author: Meng Zhang <mza117jc@gmail.com> Reviewed-by: David Rowley <dgrowleyml@gmail.com> Discussion: https://postgr.es/m/CAErYLFAuxmW0UVdgrz7iiuNrxGQnFK_OP9hBD5CUzRgjrVrz=Q@mail.gmail.com
diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c
@@ -261,13 +261,10 @@ find_window_functions_walker(Node *node, WindowFuncLists *lists)
 		if (wfunc->winref > lists->maxWinRef)
 			elog(ERROR, "WindowFunc contains out-of-range winref %u",
 				 wfunc->winref);
-		/* eliminate duplicates, so that we avoid repeated computation */
-		if (!list_member(lists->windowFuncs[wfunc->winref], wfunc))
-		{
-			lists->windowFuncs[wfunc->winref] =
-				lappend(lists->windowFuncs[wfunc->winref], wfunc);
-			lists->numWindowFuncs++;
-		}
+
+		lists->windowFuncs[wfunc->winref] =
+			lappend(lists->windowFuncs[wfunc->winref], wfunc);
+		lists->numWindowFuncs++;
 
 		/*
 		 * We assume that the parser checked that there are no window
