Функция plpgsql с курсором.

CREATE FUNCTION select_all_from_users ()  
  RETURNS TABLE (
    id  INT,  
    name VARCHAR,
    age INT,
    date_add timestamp
  )
  AS
$$
DECLARE
  users RECORD;
BEGIN
  FOR users IN SELECT * FROM customers LOOP
    RETURN NEXT;
  END LOOP;
END;
$$
LANGUAGE plpgsql;