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
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