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/primnodes.h

@@ -247,6 +247,7 @@ typedef struct Aggref
 	List	   *args;			/* arguments and sort expressions */
 	List	   *aggorder;		/* ORDER BY (list of SortGroupClause) */
 	List	   *aggdistinct;	/* DISTINCT (list of SortGroupClause) */
+	Expr	   *aggfilter;		/* FILTER expression */
 	bool		aggstar;		/* TRUE if argument list was really '*' */
 	Index		agglevelsup;	/* > 0 if agg belongs to outer query */
 	int			location;		/* token location, or -1 if unknown */
@@ -263,6 +264,7 @@ typedef struct WindowFunc
 	Oid			wincollid;		/* OID of collation of result */
 	Oid			inputcollid;	/* OID of collation that function should use */
 	List	   *args;			/* arguments to the window function */
+	Expr	   *aggfilter;		/* FILTER expression */
 	Index		winref;			/* index of associated WindowClause */
 	bool		winstar;		/* TRUE if argument list was really '*' */
 	bool		winagg;			/* is function a simple aggregate? */