MySQLexplain介绍
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）。
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）。
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 */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
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
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
mysql> explain select 1 unionallselect 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
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
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：
mysql> explain select 1 unionallselect 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 <derivenN> 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
mysql> explain selectmin(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。
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 */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。
mysql> explain select*from film_actor leftjoin 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。
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 leftjoin 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。
mysql> explain select*from film where name = "film1" or name isnull;+----+-------------+-------+-------------+---------------+----------+---------+-------+------+--------------------------+| 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
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。
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。
mysql> explain selectcount(*) 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
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。
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。
10. Extra列
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
mysql> explain selectdistinct name from film leftjoin 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。
mysql> explain select id from film orderby 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。
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。
1. actor.name没有索引，此时创建了张临时表来distinctmysql
> explain selectdistinct 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 selectdistinct 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。
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 orderby 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 orderby 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：
DROPTABLEIFEXISTS `actor`;CREATETABLE `actor` (`id`
int(11) NOTNULL,`name`
varchar(45) DEFAULTNULL,`update_time`
datetimeDEFAULTNULL,PRIMARYKEY (`id`)) ENGINE
=InnoDB DEFAULT CHARSET=utf8;INSERTINTO `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");DROPTABLEIFEXISTS `film`;CREATETABLE `film` (`id`
int(11) NOTNULL AUTO_INCREMENT,`name`
varchar(10) DEFAULTNULL,PRIMARYKEY (`id`),KEY `idx_name` (`name`)) ENGINE
=InnoDB DEFAULT CHARSET=utf8;INSERTINTO `film` (`id`, `name`) VALUES (3,"film0"),(1,"film1"),(2,"film2");DROPTABLEIFEXISTS `film_actor`;CREATETABLE `film_actor` (`id`
int(11) NOTNULL,`film_id`
int(11) NOTNULL,`actor_id`
int(11) NOTNULL,PRIMARYKEY (`id`),KEY `idx_film_actor_id` (`film_id`,`actor_id`)) ENGINE
=InnoDB DEFAULT CHARSET=utf8;INSERTINTO `film_actor` (`id`, `film_id`, `actor_id`) VALUES (1,1,1),(2,1,2),(3,2,1);
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