mysql group by优化

mysql> explain select actor.first_name,actor.last_name,count(*)  from sakila.film_actor inner join sakila.actor using(actor_id) group by film_actor.actor_id\G

*************************** 1. row ***************************

           id: 1

  select_type: SIMPLE

        table: actor

         type: ALL

possible_keys: PRIMARY

          key: NULL

      key_len: NULL

          ref: NULL

         rows: 200

        Extra: Using temporary; Using filesort

*************************** 2. row ***************************

           id: 1

  select_type: SIMPLE

        table: film_actor

         type: ref

possible_keys: PRIMARY,idx_fk_film_id

          key: PRIMARY

      key_len: 2

          ref: sakila.actor.actor_id

         rows: 1

        Extra: Using index

2 rows in set (0.00 sec)



mysql> 

从explain看，上面的写法使用了临时表和文件排序

改写后

mysql> explain select actor.first_name,actor.last_name,c.cnt from sakila.actor inner join (select actor_id,count(*) as cnt from sakila.film_actor group by actor_id) as c using(actor_id)\G

*************************** 1. row ***************************

           id: 1

  select_type: PRIMARY

        table: actor

         type: ALL

possible_keys: PRIMARY

          key: NULL

      key_len: NULL

          ref: NULL

         rows: 200

        Extra: NULL

*************************** 2. row ***************************

           id: 1

  select_type: PRIMARY

        table: <derived2>

         type: ref

possible_keys: <auto_key0>

          key: <auto_key0>

      key_len: 2

          ref: sakila.actor.actor_id

         rows: 27

        Extra: NULL

*************************** 3. row ***************************

           id: 2

  select_type: DERIVED

        table: film_actor

         type: index

possible_keys: PRIMARY,idx_fk_film_id

          key: PRIMARY

      key_len: 4

          ref: NULL

         rows: 5462

        Extra: Using index

3 rows in set (0.00 sec)



mysql> 

 

如果是有过滤条件的子查询，查询过滤条件尽量加到子查询条件中，而不要加到外面