如何首先运行子查询并且只运行一次

Question

如何让MySQL首先运行子查询并且只运行一次？ MySQL现在为表t1中的每一行运行内部查询，这是性能灾难。

explain select * from t1 where uid in (select id from t0); 
+----+--------------------+-------+------+---------------+------+---------+------+----------+-------------+ 
| id | select_type  | table | type | possible_keys | key | key_len | ref | rows  | Extra  | 
+----+--------------------+-------+------+---------------+------+---------+------+----------+-------------+ 
| 1 | PRIMARY   | t1 | ALL | NULL   | NULL | NULL | NULL | 18954249 | Using where | 
| 2 | DEPENDENT SUBQUERY | t0 | ALL | NULL   | NULL | NULL | NULL | 12749 | Using where | 
+----+--------------------+-------+------+---------------+------+---------+------+----------+-------------+ 

Answer 1

如果您执行inner join，您会得到期望的结果。

来源：

select * from t1 where uid in (select id from t0); 

要：

select * from t1 join t0 on t1.t0id = t0.id 

http://en.wikipedia.org/wiki/Join_(SQL)#Inner_join

Answer 2

以下2将工作速度比你目前的query，哪一个能更好地伸缩取决于你的数据库结构

SELECT 
    * 
FROM 
    t1 t 
    JOIN t0 ON (t.uid = t0.id)  << If there are 2 row matches on table `t0`, it shall return duplicated rows (t1 table contents duplicated, you could use a distinct to avoid such cases) OR use the second query instead 

和

SELECT 
    * 
FROM 
    t1 t 
    WHERE EXISTS (select 1 from t0 WHERE t0.id = t.uid)