Implement the FILTER clause for aggregate function calls.

This is SQL-standard with a few extensions, namely support for
subqueries and outer references in clause expressions.

catversion bump due to change in Aggref and WindowFunc.

David Fetter, reviewed by Dean Rasheed.

src/include/nodes/parsenodes.h

@@ -283,8 +283,8 @@ typedef struct CollateClause
  * agg_star indicates we saw a 'foo(*)' construct, while agg_distinct
  * indicates we saw 'foo(DISTINCT ...)'.  In any of these cases, the
  * construct *must* be an aggregate call.  Otherwise, it might be either an
- * aggregate or some other kind of function.  However, if OVER is present
- * it had better be an aggregate or window function.
+ * aggregate or some other kind of function.  However, if FILTER or OVER is
+ * present it had better be an aggregate or window function.
  *
  * Normally, you'd initialize this via makeFuncCall() and then only
  * change the parts of the struct its defaults don't match afterwards
@@ -297,6 +297,7 @@ typedef struct FuncCall
 	List	   *funcname;		/* qualified name of function */
 	List	   *args;			/* the arguments (list of exprs) */
 	List	   *agg_order;		/* ORDER BY (list of SortBy) */
+	Node	   *agg_filter;		/* FILTER clause, if any */
 	bool		agg_star;		/* argument was really '*' */
 	bool		agg_distinct;	/* arguments were labeled DISTINCT */
 	bool		func_variadic;	/* last argument was labeled VARIADIC */