请问如下子查询的优化，为什么性能变差。

如果是两个表之间的连接查询：

select film_id from film_actor where actor_id in (select actor_id from actor where first_name = 'sandra')

select film_id from film_actor join actor on (film_actor.actor_id = actor.actor_id and actor.first_name = 'sandra')

这里，join查询性能优于子查询， 比较好理解。性能如下：

子查询：   Query_time: 0.008001  Lock_time: 0.000000 Rows_sent: 56  Rows_examined: 5462

join查询： Query_time: 0.000996  Lock_time: 0.000996 Rows_sent: 56  Rows_examined: 256

但3个表的子查询：

课件中的子查询：

select title, release_year, length from film where film_id in(select film_id from film_actor where actor_id in (select actor_id from actor where first_name = 'sandra'));

我自己写的连接查询：

select title, release_year, length from film join film_actor join actor on (film.film_id = film_actor.film_id and film_actor.actor_id = actor.actor_id and actor.first_name = 'sandra');

执行日志：

子查询：    Query_time: 0.005999  Lock_time: 0.000000 Rows_sent: 56  Rows_examined: 1000

join查询： Query_time: 0.008032  Lock_time: 0.000000 Rows_sent: 56  Rows_examined: 11924

请问，为什么这里，子查询的性能要优于连接查询？