explain select actor.first_name, actor.last_name, count(*) from (film_actor inner join actor on film_actor.actor_id = actor.actor_id) group by actor.actor_id\G; 与这个相比， 我的这种写法的分析结果其实更好一点，区别就在于最后group by 的actor_id是用的actor的， 结果如下， 请问为何会这样。

+----+-------------+------------+-------+---------------+---------+---------+-----------------------+------+-------------+

| id | select_type | table      | type  | possible_keys | key     | key_len | ref                   | rows | Extra       |

+----+-------------+------------+-------+---------------+---------+---------+-----------------------+------+-------------+

|  1 | SIMPLE      | actor      | index | PRIMARY       | PRIMARY | 2       | NULL                  |   15 |             |

|  1 | SIMPLE      | film_actor | ref   | PRIMARY       | PRIMARY | 2       | sakila.actor.actor_id |   13 | Using index |

+----+-------------+------------+-------+---------------+---------+---------+-----------------------+------+-------------+