DO $$
   BEGIN
   FOR i IN 1 .. 1000 LOOP
      BEGIN
         LOCK TABLE t NOWAIT;
         ALTER TABLE t ADD COLUMN i BIGINT;
         RETURN;
      EXCEPTION WHEN lock_not_available THEN
         PERFORM pg_sleep(1);
         CONTINUE;
      END;
   END LOOP;
   RAISE lock_not_available;
   END;
$$;