1. delete limit
      1. -- 该语法pg不支持
      2. delete from test where true order by id limit 1;
      3. -- pg的等价语法, ctid是隐藏列, ctid在vacuum full执行后会变动
      4. -- 但在一条语句/一个事务用来限制delete是安全的
      5. delete from test where ctid in (select ctid from test order by c1 limit 1);