delete执行速度优化

现象：delete子查询非常慢，但select非常快，如下：

select * from test1 where uid in (select uid from test2)-- 非常快

delete from test1 where uid in (select uid from test2)--非常慢

改成join表就很快了

delete t1 from test1 t1 inner join test2 t2 on t1.uid=t2.uid