3 files changed, 55 insertions, 13 deletions

diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c
index 76c032c5698..4fa73a9d223 100644
--- a/src/backend/optimizer/util/clauses.c
+++ b/src/backend/optimizer/util/clauses.c
@@ -2251,6 +2251,56 @@ eval_const_expressions_mutator(Node *node,
 				 */
 				return (Node *) copyObject(param);
 			}
+		case T_WindowFunc:
+			{
+				WindowFunc *expr = (WindowFunc *) node;
+				Oid			funcid = expr->winfnoid;
+				List	   *args;
+				Expr	   *aggfilter;
+				HeapTuple	func_tuple;
+				WindowFunc *newexpr;
+
+				/*
+				 * We can't really simplify a WindowFunc node, but we mustn't
+				 * just fall through to the default processing, because we
+				 * have to apply expand_function_arguments to its argument
+				 * list.  That takes care of inserting default arguments and
+				 * expanding named-argument notation.
+				 */
+				func_tuple = SearchSysCache1(PROCOID, ObjectIdGetDatum(funcid));
+				if (!HeapTupleIsValid(func_tuple))
+					elog(ERROR, "cache lookup failed for function %u", funcid);
+
+				args = expand_function_arguments(expr->args, expr->wintype,
+												 func_tuple);
+
+				ReleaseSysCache(func_tuple);
+
+				/* Now, recursively simplify the args (which are a List) */
+				args = (List *)
+					expression_tree_mutator((Node *) args,
+											eval_const_expressions_mutator,
+											(void *) context);
+				/* ... and the filter expression, which isn't */
+				aggfilter = (Expr *)
+					eval_const_expressions_mutator((Node *) expr->aggfilter,
+												   context);
+
+				/* And build the replacement WindowFunc node */
+				newexpr = makeNode(WindowFunc);
+				newexpr->winfnoid = expr->winfnoid;
+				newexpr->wintype = expr->wintype;
+				newexpr->wincollid = expr->wincollid;
+				newexpr->inputcollid = expr->inputcollid;
+				newexpr->args = args;
+				newexpr->aggfilter = aggfilter;
+				newexpr->winref = expr->winref;
+				newexpr->winstar = expr->winstar;
+				newexpr->winagg = expr->winagg;
+				newexpr->location = expr->location;
+
+				return (Node *) newexpr;
+			}
 		case T_FuncExpr:
 			{
 				FuncExpr   *expr = (FuncExpr *) node;
diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index 2bd24c89c87..ede36d159a3 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -537,17 +537,6 @@ ParseFuncOrColumn(ParseState *pstate, List *funcname, List *fargs,
 					 errmsg("window functions cannot return sets"),
 					 parser_errposition(pstate, location)));
 
-		/*
-		 * We might want to support this later, but for now reject it because
-		 * the planner and executor wouldn't cope with NamedArgExprs in a
-		 * WindowFunc node.
-		 */
-		if (argnames != NIL)
-			ereport(ERROR,
-					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("window functions cannot use named arguments"),
-					 parser_errposition(pstate, location)));
-
 		/* parse_agg.c does additional window-func-specific processing */
 		transformWindowFuncCall(pstate, wfunc, over);
 
diff --git a/src/backend/utils/adt/ruleutils.c b/src/backend/utils/adt/ruleutils.c
index 04b1c4f8956..915fb7a6689 100644
--- a/src/backend/utils/adt/ruleutils.c
+++ b/src/backend/utils/adt/ruleutils.c
@@ -7461,6 +7461,7 @@ get_windowfunc_expr(WindowFunc *wfunc, deparse_context *context)
 	StringInfo	buf = context->buf;
 	Oid			argtypes[FUNC_MAX_ARGS];
 	int			nargs;
+	List	   *argnames;
 	ListCell   *l;
 
 	if (list_length(wfunc->args) > FUNC_MAX_ARGS)
@@ -7468,18 +7469,20 @@ get_windowfunc_expr(WindowFunc *wfunc, deparse_context *context)
 				(errcode(ERRCODE_TOO_MANY_ARGUMENTS),
 				 errmsg("too many arguments")));
 	nargs = 0;
+	argnames = NIL;
 	foreach(l, wfunc->args)
 	{
 		Node	   *arg = (Node *) lfirst(l);
 
-		Assert(!IsA(arg, NamedArgExpr));
+		if (IsA(arg, NamedArgExpr))
+			argnames = lappend(argnames, ((NamedArgExpr *) arg)->name);
 		argtypes[nargs] = exprType(arg);
 		nargs++;
 	}
 
 	appendStringInfo(buf, "%s(",
 					 generate_function_name(wfunc->winfnoid, nargs,
-											NIL, argtypes,
+											argnames, argtypes,
 											false, NULL));
 	/* winstar can be set only in zero-argument aggregates */
 	if (wfunc->winstar)