Support default arguments and named-argument notation for window functions.

These things didn't work because the planner omitted to do the necessary
preprocessing of a WindowFunc's argument list.  Add the few dozen lines
of code needed to handle that.

Although this sounds like a feature addition, it's really a bug fix because
the default-argument case was likely to crash previously, due to lack of
checking of the number of supplied arguments in the built-in window
functions.  It's not a security issue because there's no way for a
non-superuser to create a window function definition with defaults that
refers to a built-in C function, but nonetheless people might be annoyed
that it crashes rather than producing a useful error message.  So
back-patch as far as the patch applies easily, which turns out to be 9.2.
I'll put a band-aid in earlier versions as a separate patch.

(Note that these features still don't work for aggregates, and fixing that
case will be harder since we represent aggregate arg lists as target lists
not bare expression lists.  There's no crash risk though because CREATE
AGGREGATE doesn't accept defaults, and we reject named-argument notation
when parsing an aggregate call.)

doc/src/sgml/syntax.sgml

@@ -2533,8 +2533,9 @@ SELECT concat_lower_or_upper('Hello', 'World', uppercase := true);
 
    <note>
     <para>
-     Named and mixed call notations can currently be used only with regular
-     functions, not with aggregate functions or window functions.
+     Named and mixed call notations currently cannot be used when calling an
+     aggregate function (but they do work when an aggregate function is used
+     as a window function).
     </para>
    </note>
   </sect2>

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;

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);
 

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)

src/test/regress/expected/window.out

@@ -1035,3 +1035,38 @@ FROM empsalary GROUP BY depname;
 
 -- cleanup
 DROP TABLE empsalary;
+-- test user-defined window function with named args and default args
+CREATE FUNCTION nth_value_def(val anyelement, n integer = 1) RETURNS anyelement
+  LANGUAGE internal WINDOW IMMUTABLE STRICT AS 'window_nth_value';
+SELECT nth_value_def(n := 2, val := ten) OVER (PARTITION BY four), ten, four
+  FROM (SELECT * FROM tenk1 WHERE unique2 < 10 ORDER BY four, ten) s;
+ nth_value_def | ten | four 
+---------------+-----+------
+             0 |   0 |    0
+             0 |   0 |    0
+             0 |   4 |    0
+             1 |   1 |    1
+             1 |   1 |    1
+             1 |   7 |    1
+             1 |   9 |    1
+               |   0 |    2
+             3 |   1 |    3
+             3 |   3 |    3
+(10 rows)
+
+SELECT nth_value_def(ten) OVER (PARTITION BY four), ten, four
+  FROM (SELECT * FROM tenk1 WHERE unique2 < 10 ORDER BY four, ten) s;
+ nth_value_def | ten | four 
+---------------+-----+------
+             0 |   0 |    0
+             0 |   0 |    0
+             0 |   4 |    0
+             1 |   1 |    1
+             1 |   1 |    1
+             1 |   7 |    1
+             1 |   9 |    1
+             0 |   0 |    2
+             1 |   1 |    3
+             1 |   3 |    3
+(10 rows)
+

src/test/regress/sql/window.sql

@@ -274,3 +274,13 @@ FROM empsalary GROUP BY depname;
 
 -- cleanup
 DROP TABLE empsalary;
+
+-- test user-defined window function with named args and default args
+CREATE FUNCTION nth_value_def(val anyelement, n integer = 1) RETURNS anyelement
+  LANGUAGE internal WINDOW IMMUTABLE STRICT AS 'window_nth_value';
+
+SELECT nth_value_def(n := 2, val := ten) OVER (PARTITION BY four), ten, four
+  FROM (SELECT * FROM tenk1 WHERE unique2 < 10 ORDER BY four, ten) s;
+
+SELECT nth_value_def(ten) OVER (PARTITION BY four), ten, four
+  FROM (SELECT * FROM tenk1 WHERE unique2 < 10 ORDER BY four, ten) s;