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diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 89ca4e08bf1..5a221214b21 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -103,6 +103,7 @@
 #include "optimizer/placeholder.h"
 #include "optimizer/plancat.h"
 #include "optimizer/restrictinfo.h"
+#include "optimizer/rpr.h"
 #include "parser/parsetree.h"
 #include "utils/lsyscache.h"
 #include "utils/selfuncs.h"
@@ -3227,12 +3228,16 @@ cost_windowagg(Path *path, PlannerInfo *root,
 	 * many rows the window function will fetch, it's hard to do better.  In
 	 * any case, it's a good estimate for all the built-in window functions,
 	 * so we'll just do this for now.
+	 *
+	 * Moreover, if DEFINE/PATTERN clause exists, we charge their expressions
+	 * per tuple.
 	 */
 	foreach(lc, windowFuncs)
 	{
 		WindowFunc *wfunc = lfirst_node(WindowFunc, lc);
 		Cost		wfunccost;
 		QualCost	argcosts;
+		QualCost	defcosts;
 
 		argcosts.startup = argcosts.per_tuple = 0;
 		add_function_cost(root, wfunc->winfnoid, (Node *) wfunc,
@@ -3245,6 +3250,37 @@ cost_windowagg(Path *path, PlannerInfo *root,
 		startup_cost += argcosts.startup;
 		wfunccost += argcosts.per_tuple;
 
+		/* also add DEFINE clause expressions' cost to per-input-row costs */
+		if (winclause->rpPattern)
+		{
+			List	   *pattern_vars;	/* list of pattern variable names */
+			ListCell   *lc2;
+
+			pattern_vars = collectPatternVariables(winclause->rpPattern);
+
+			/* iterate according to the pattern variable */
+			foreach(lc2, pattern_vars)
+			{
+				char	   *ptname = strVal((char *) lfirst(lc2));
+
+				/* iterate according to the DEFINE clause */
+				foreach_node(TargetEntry, def, winclause->defineClause)
+				{
+					if (!strcmp(ptname, def->resname))
+					{
+						/*
+						 * varname found. Add DEFINE clause expressions' cost
+						 * to per-input-row costs.
+						 */
+						cost_qual_eval_node(&defcosts, (Node *) def->expr, root);
+						startup_cost += defcosts.startup;
+						wfunccost += defcosts.per_tuple;
+					}
+				}
+			}
+			list_free_deep(pattern_vars);
+		}
+
 		/*
 		 * Add the filter's cost to per-input-row costs.  XXX We should reduce
 		 * input expression costs according to filter selectivity.