gistfile1.txt

ishida=# create function t(bool) returns bool as 'select true' language sql;
CREATE FUNCTION
ishida=# create aggregate exists_(*) (sfunc = t, stype = bool, initcond = false);
CREATE AGGREGATE

ishida=# select exists_(*) from pg_user;
 exists_
---------
 t
(1 row)

ishida=# select exists_(*) from pg_user where usename = 'hoge';
 exists_
---------
 f
(1 row)