原文地址 https://cloud.tencent.com/developer/article/1093229

Explain 简介

本文主要讲述如何通过 explain 命令获取 select 语句的执行计划,通过 explain 我们可以知道以下信息:表的读取顺序,数据读取操作的类型,哪些索引可以使用,哪些索引实际使用了,表之间的引用,每张表有多少行被优化器查询等信息。

下面是使用 explain 的例子:

在 select 语句之前增加 explain 关键字,MySQL会在查询上设置一个标记,执行查询时,会返回执行计划的信息,而不是执行这条 SQL(如果 from 中包含子查询,仍会执行该子查询,将结果放入临时表中)。

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mysql> explain select * from actor;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| 1 | SIMPLE | actor | ALL | NULL | NULL | NULL | NULL | 2 | NULL |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+

在查询中的每个表会输出一行,如果有两个表通过 join 连接查询,那么会输出两行。表的意义相当广泛:可以是子查询、一个 union 结果等。

explain 有两个变种:

1)explain extended:会在 explain  的基础上额外提供一些查询优化的信息。紧随其后通过 show warnings 命令可以 得到优化后的查询语句,从而看出优化器优化了什么。额外还有 filtered 列,是一个半分比的值,rows * filtered/100 可以估算出将要和 explain 中前一个表进行连接的行数(前一个表指 explain 中的 id 值比当前表 id 值小的表)。

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mysql> explain extended select * from film where id = 1;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | film | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+----------+-------+

mysql> show warnings;
+-------+------+--------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+--------------------------------------------------------------------------------+
| Note | 1003 | select '1' AS `id`,'film1' AS `name` from `test`.`film` where 1 |
+-------+------+--------------------------------------------------------------------------------+

2)explain partitions:相比 explain 多了个 partitions 字段,如果查询是基于分区表的话,会显示查询将访问的分区。

explain 中的列

接下来我们将展示 explain 中每个列的信息。

1. id 列

id 列的编号是 select 的序列号,有几个 select 就有几个 id,并且 id 的顺序是按 select 出现的顺序增长的。MySQL 将 select 查询分为简单查询和复杂查询。复杂查询分为三类:简单子查询、派生表(from 语句中的子查询)、union 查询。

1)简单子查询

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mysql> explain select (select 1 from actor limit 1) from film;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| 1 | PRIMARY | film | index | NULL | idx_name | 32 | NULL | 1 | Using index |
| 2 | SUBQUERY | actor | index | NULL | PRIMARY | 4 | NULL | 2 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

2)from 子句中的子查询

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mysql> explain select id from (select id from film) as der;
+----+-------------+------------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+-------+---------------+----------+---------+------+------+-------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 2 | NULL |
| 2 | DERIVED | film | index | NULL | idx_name | 32 | NULL | 1 | Using index |
+----+-------------+------------+-------+---------------+----------+---------+------+------+-------------+

这个查询执行时有个临时表别名为 der,外部 select 查询引用了这个临时表

3)union 查询

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mysql> explain select 1 union all select 1;
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
| 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
| 2 | UNION | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
| NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+

union 结果总是放在一个匿名临时表中,临时表不在 SQL 总出现,因此它的 id 是 NULL。

2. select_type 列

select_type 表示对应行是是简单还是复杂的查询,如果是复杂的查询,又是上述三种复杂查询中的哪一种。

1)simple:简单查询。查询不包含子查询和 union

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mysql> explain select * from film where id = 2;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | film | const | PRIMARY | PRIMARY | 4 | const | 1 | NULL |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+

2)primary:复杂查询中最外层的 select

3)subquery:包含在 select 中的子查询(不在 from 子句中)

4)derived:包含在 from 子句中的子查询。MySQL 会将结果存放在一个临时表中,也称为派生表(derived 的英文含义)

用这个例子来了解 primary、subquery 和 derived 类型

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mysql> explain select (select 1 from actor where id = 1) from (select * from film where id = 1) der;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+
| 1 | PRIMARY | <derived3> | system | NULL | NULL | NULL | NULL | 1 | NULL |
| 3 | DERIVED | film | const | PRIMARY | PRIMARY | 4 | const | 1 | NULL |
| 2 | SUBQUERY | actor | const | PRIMARY | PRIMARY | 4 | const | 1 | Using index |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+

5)union:在 union 中的第二个和随后的 select

6)union result:从 union 临时表检索结果的 select

用这个例子来了解 union 和 union result 类型:

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mysql> explain select 1 union all select 1;
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
| 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
| 2 | UNION | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
| NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+

3. table 列

这一列表示 explain 的一行正在访问哪个表。

当 from 子句中有子查询时,table 列是 格式,表示当前查询依赖 id=N 的查询,于是先执行 id=N 的查询。当有 union 时,UNION RESULT 的 table 列的值为 <union1,2>,1 和 2 表示参与 union 的 select 行 id。

4. type 列

这一列表示关联类型或访问类型,即 MySQL 决定如何查找表中的行。

依次从最优到最差分别为:system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL

NULL:mysql 能够在优化阶段分解查询语句,在执行阶段用不着再访问表或索引。例如:在索引列中选取最小值,可以单独查找索引来完成,不需要在执行时访问表

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mysql> explain select min(id) from film;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+

**const, system**:mysql 能对查询的某部分进行优化并将其转化成一个常量(可以看 show warnings 的结果)。用于 primary key 或 unique key 的所有列与常数比较时,所以表最多有一个匹配行,读取 1 次,速度比较快。

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mysql> explain extended select * from (select * from film where id = 1) tmp;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+
| 1 | PRIMARY | <derived2> | system | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL |
| 2 | DERIVED | film | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+

mysql> show warnings;
+-------+------+---------------------------------------------------------------+
| Level | Code | Message |
+-------+------+---------------------------------------------------------------+
| Note | 1003 | select '1' AS `id`,'film1' AS `name` from dual |
+-------+------+---------------------------------------------------------------+

eq_ref:primary key 或 unique key 索引的所有部分被连接使用 ,最多只会返回一条符合条件的记录。这可能是在 const 之外最好的联接类型了,简单的 select 查询不会出现这种 type。

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mysql> explain select * from film_actor left join film on film_actor.film_id = film.id;
+----+-------------+------------+--------+---------------+-------------------+---------+-------------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+---------------+-------------------+---------+-------------------------+------+-------------+
| 1 | SIMPLE | film_actor | index | NULL | idx_film_actor_id | 8 | NULL | 3 | Using index |
| 1 | SIMPLE | film | eq_ref | PRIMARY | PRIMARY | 4 | test.film_actor.film_id | 1 | NULL |
+----+-------------+------------+--------+---------------+-------------------+---------+-------------------------+------+-------------+

**ref**:相比 eq_ref,不使用唯一索引,而是使用普通索引或者唯一性索引的部分前缀,索引要和某个值相比较,可能会找到多个符合条件的行。

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1. 简单 select 查询,name是普通索引(非唯一索引)
mysql> explain select * from film where name = "film1";
+----+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+
| 1 | SIMPLE | film | ref | idx_name | idx_name | 33 | const | 1 | Using where; Using index |
+----+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+

2.关联表查询,idx_film_actor_id是film_id和actor_id的联合索引,这里使用到了film_actor的左边前缀film_id部分。
mysql> explain select * from film left join film_actor on film.id = film_actor.film_id;
+----+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+-------------+
| 1 | SIMPLE | film | index | NULL | idx_name | 33 | NULL | 3 | Using index |
| 1 | SIMPLE | film_actor | ref | idx_film_actor_id | idx_film_actor_id | 4 | test.film.id | 1 | Using index |
+----+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+-------------+

**ref_or_null**:类似ref,但是可以搜索值为 NULL 的行。

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mysql> explain select * from film where name = "film1" or name is null;
+----+-------------+-------+-------------+---------------+----------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------------+---------------+----------+---------+-------+------+--------------------------+
| 1 | SIMPLE | film | ref_or_null | idx_name | idx_name | 33 | const | 2 | Using where; Using index |
+----+-------------+-------+-------------+---------------+----------+---------+-------+------+--------------------------+

**index_merge**:表示使用了索引合并的优化方法。 例如下表:id 是主键,tenant_id 是普通索引。or 的时候没有用 primary key,而是使用了 primary key(id) 和 tenant_id 索引

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mysql> explain select * from role where id = 11011 or tenant_id = 8888;
+----+-------------+-------+-------------+-----------------------+-----------------------+---------+------+------+-------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------------+-----------------------+-----------------------+---------+------+------+-------------------------------------------------+
| 1 | SIMPLE | role | index_merge | PRIMARY,idx_tenant_id | PRIMARY,idx_tenant_id | 4,4 | NULL | 134 | Using union(PRIMARY,idx_tenant_id); Using where |
+----+-------------+-------+-------------+-----------------------+-----------------------+---------+------+------+-------------------------------------------------+

**range**:范围扫描通常出现在 in(), between ,> ,<, >= 等操作中。使用一个索引来检索给定范围的行。

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mysql> explain select * from actor where id > 1;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | actor | range | PRIMARY | PRIMARY | 4 | NULL | 2 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+

**index**:和 ALL 一样,不同就是 mysql 只需扫描索引树,这通常比 ALL 快一些。

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mysql> explain select count(*) from film;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| 1 | SIMPLE | film | index | NULL | idx_name | 33 | NULL | 3 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

**ALL**:即全表扫描,意味着 mysql 需要从头到尾去查找所需要的行。通常情况下这需要增加索引来进行优化了

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mysql> explain select * from actor;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| 1 | SIMPLE | actor | ALL | NULL | NULL | NULL | NULL | 2 | NULL |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+

5. possible_keys 列

这一列显示查询可能使用哪些索引来查找。

explain 时可能出现 possible_keys 有列,而 key 显示 NULL 的情况,这种情况是因为表中数据不多,mysql 认为索引对此查询帮助不大,选择了全表查询。

如果该列是 NULL,则没有相关的索引。在这种情况下,可以通过检查 where 子句看是否可以创造一个适当的索引来提高查询性能,然后用 explain 查看效果。

6. key 列

这一列显示 mysql 实际采用哪个索引来优化对该表的访问。

如果没有使用索引,则该列是 NULL。如果想强制 mysql 使用或忽视 possible_keys 列中的索引,在查询中使用 force index、ignore index。

7. key_len 列

这一列显示了 mysql 在索引里使用的字节数,通过这个值可以算出具体使用了索引中的哪些列。

举例来说,film_actor 的联合索引 idx_film_actor_id 由 film_id 和 actor_id 两个 int 列组成,并且每个 int 是 4 字节。通过结果中的 key_len=4 可推断出查询使用了第一个列:film_id 列来执行索引查找。

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mysql> explain select * from film_actor where film_id = 2;
+----+-------------+------------+------+-------------------+-------------------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+------+-------------------+-------------------+---------+-------+------+-------------+
| 1 | SIMPLE | film_actor | ref | idx_film_actor_id | idx_film_actor_id | 4 | const | 1 | Using index |
+----+-------------+------------+------+-------------------+-------------------+---------+-------+------+-------------+

key_len 计算规则如下:

  • 字符串
    • char(n):n 字节长度
    • varchar(n):2 字节存储字符串长度,如果是 utf-8,则长度 3n + 2
  • 数值类型
    • tinyint:1 字节
    • smallint:2 字节
    • int:4 字节
    • bigint:8 字节  
  • 时间类型 
    • date:3 字节
    • timestamp:4 字节
    • datetime:8 字节
  • 如果字段允许为 NULL,需要 1 字节记录是否为 NULL

索引最大长度是 768 字节,当字符串过长时,mysql 会做一个类似左前缀索引的处理,将前半部分的字符提取出来做索引。

8. ref 列

这一列显示了在 key 列记录的索引中,表查找值所用到的列或常量,常见的有:const(常量),func,NULL,字段名(例:film.id)

9. rows 列

这一列是 mysql 估计要读取并检测的行数,注意这个不是结果集里的行数。

这一列展示的是额外信息。常见的重要值如下:

distinct: 一旦 mysql 找到了与行相联合匹配的行,就不再搜索了

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mysql> explain select distinct name from film left join film_actor on film.id = film_actor.film_id;
+----+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+------------------------------+
| 1 | SIMPLE | film | index | idx_name | idx_name | 33 | NULL | 3 | Using index; Using temporary |
| 1 | SIMPLE | film_actor | ref | idx_film_actor_id | idx_film_actor_id | 4 | test.film.id | 1 | Using index; Distinct |
+----+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+------------------------------+

**Using index**:这发生在对表的请求列都是同一索引的部分的时候,返回的列数据只使用了索引中的信息,而没有再去访问表中的行记录。是性能高的表现。

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mysql> explain select id from film order by id;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | film | index | NULL | PRIMARY | 4 | NULL | 3 | Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+

**Using where**:mysql 服务器将在存储引擎检索行后再进行过滤。就是先读取整行数据,再按 where 条件进行检查,符合就留下,不符合就丢弃。

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mysql> explain select * from film where id > 1;
+----+-------------+-------+-------+---------------+----------+---------+------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------+------+--------------------------+
| 1 | SIMPLE | film | index | PRIMARY | idx_name | 33 | NULL | 3 | Using where; Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+--------------------------+

**Using temporary**:mysql 需要创建一张临时表来处理查询。出现这种情况一般是要进行优化的,首先是想到用索引来优化。

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1. actor.name没有索引,此时创建了张临时表来distinct
mysql> explain select distinct name from actor;
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------+
| 1 | SIMPLE | actor | ALL | NULL | NULL | NULL | NULL | 2 | Using temporary |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------+

2. film.name建立了idx_name索引,此时查询时extra是using index,没有用临时表
mysql> explain select distinct name from film;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| 1 | SIMPLE | film | index | idx_name | idx_name | 33 | NULL | 3 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

**Using filesort**:mysql 会对结果使用一个外部索引排序,而不是按索引次序从表里读取行。此时 mysql 会根据联接类型浏览所有符合条件的记录,并保存排序关键字和行指针,然后排序关键字并按顺序检索行信息。这种情况下一般也是要考虑使用索引来优化的。

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1. actor.name未创建索引,会浏览actor整个表,保存排序关键字name和对应的id,然后排序name并检索行记录
mysql> explain select * from actor order by name;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| 1 | SIMPLE | actor | ALL | NULL | NULL | NULL | NULL | 2 | Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+

2. film.name建立了idx_name索引,此时查询时extra是using index
mysql> explain select * from film order by name;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| 1 | SIMPLE | film | index | NULL | idx_name | 33 | NULL | 3 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

使用的表

以上所有 sql 使用的表和数据:

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DROP TABLE IF EXISTS `actor`;
CREATE TABLE `actor` (
`id` int(11) NOT NULL,
`name` varchar(45) DEFAULT NULL,
`update_time` datetime DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `actor` (`id`, `name`, `update_time`) VALUES (1,'a','2017-12-22 15:27:18'), (2,'b','2017-12-22 15:27:18'), (3,'c','2017-12-22 15:27:18');

DROP TABLE IF EXISTS `film`;
CREATE TABLE `film` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(10) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `idx_name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `film` (`id`, `name`) VALUES (3,'film0'),(1,'film1'),(2,'film2');

DROP TABLE IF EXISTS `film_actor`;
CREATE TABLE `film_actor` (
`id` int(11) NOT NULL,
`film_id` int(11) NOT NULL,
`actor_id` int(11) NOT NULL,
PRIMARY KEY (`id`),
KEY `idx_film_actor_id` (`film_id`,`actor_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `film_actor` (`id`, `film_id`, `actor_id`) VALUES (1,1,1),(2,1,2),(3,2,1);

参考