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/parser/kwlist.h

@@ -155,6 +155,7 @@ PG_KEYWORD("extract", EXTRACT, COL_NAME_KEYWORD)
 PG_KEYWORD("false", FALSE_P, RESERVED_KEYWORD)
 PG_KEYWORD("family", FAMILY, UNRESERVED_KEYWORD)
 PG_KEYWORD("fetch", FETCH, RESERVED_KEYWORD)
+PG_KEYWORD("filter", FILTER, UNRESERVED_KEYWORD)
 PG_KEYWORD("first", FIRST_P, UNRESERVED_KEYWORD)
 PG_KEYWORD("float", FLOAT_P, COL_NAME_KEYWORD)
 PG_KEYWORD("following", FOLLOWING, UNRESERVED_KEYWORD)

src/include/parser/parse_func.h

@@ -42,10 +42,9 @@ typedef enum
 } FuncDetailCode;
 
 
-extern Node *ParseFuncOrColumn(ParseState *pstate,
-				  List *funcname, List *fargs,
-				  List *agg_order, bool agg_star, bool agg_distinct,
-				  bool func_variadic,
+extern Node *ParseFuncOrColumn(ParseState *pstate, List *funcname, List *fargs,
+				  List *agg_order, Expr *agg_filter,
+				  bool agg_star, bool agg_distinct, bool func_variadic,
 				  WindowDef *over, bool is_column, int location);
 
 extern FuncDetailCode func_get_detail(List *funcname,

src/include/parser/parse_node.h

@@ -39,6 +39,7 @@ typedef enum ParseExprKind
 	EXPR_KIND_FROM_FUNCTION,	/* function in FROM clause */
 	EXPR_KIND_WHERE,			/* WHERE */
 	EXPR_KIND_HAVING,			/* HAVING */
+	EXPR_KIND_FILTER,			/* FILTER */
 	EXPR_KIND_WINDOW_PARTITION, /* window definition PARTITION BY */
 	EXPR_KIND_WINDOW_ORDER,		/* window definition ORDER BY */
 	EXPR_KIND_WINDOW_FRAME_RANGE,		/* window frame clause with RANGE */