CREATE OR REPLACE FUNCTION to_json(text, VARIADIC text[]) 
RETURNS text AS
$BODY$
DECLARE
 RESULT text;
 SQL text;
 COL text;
BEGIN
 SQL = $1;
 COL = array_to_string($2, '');

EXECUTE 'SELECT json_agg(row_to_json((SELECT COL FROM (SELECT '||COL||') AS COL))) FROM ('||SQL||') AS D' INTO RESULT;
RETURN RESULT;
END;
$BODY$
LANGUAGE plpgsql;

SELECT to_json('SELECT * FROM test1','id, caption')

SELECT to_json('SELECT * FROM test1','id, caption, ((SELECT to_json('||'SELECT * FROM TEST2 where test2.test1_id=d.id'||', '||'id, test1_id, caption'||') )) ')