谈谈关于MySQL explain 的详解

​

 ​​​

当咱们写下一条复杂的sql时，不是盲目去执行，选择提早explain分析sql，是一个不错的选择，避免没用到索引或者用到错误的索引致使mysql大量的扫表，形成线上很差的后果。
mysql提供的explain命令能够获取select语句的执行计划，经过explain咱们能够知道：表的读取顺序，数据读取操做的类型，哪些索引可使用，哪些索引实际使用了等等。

mysql> explain select * from user;
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | user  | NULL       | index | NULL          | name | 402     | NULL |    1 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)

如下所有基于mysql5.7.32

mysql> select version();
+-----------+
| version() |
+-----------+
| 5.7.32    |
+-----------+
1 row in set (0.00 sec)

id

mysql> explain select * from user a left join user_info b on a.id=b.user_id where a.id=1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | a     | NULL       | const | PRIMARY       | PRIMARY | 8       | const |    1 |   100.00 | NULL  |
|  1 | SIMPLE      | b     | NULL       | ref   | user_id       | user_id | 8       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+

id相同的时候，从上往下顺序执行

mysql> explain select * from user where id = (select user_id from user_info where age=10);
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table     | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
|  1 | PRIMARY     | user      | NULL       | const | PRIMARY       | PRIMARY | 8       | const |    1 |   100.00 | NULL        |
|  2 | SUBQUERY    | user_info | NULL       | ALL   | NULL          | NULL    | NULL    | NULL  |    1 |   100.00 | Using where |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+

id不一样的时候，越大的id越先执行，对于这种子查询，优先执行子sql拿到结果，才去执行主sql

mysql> explain select * from user  union  select * from user;
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type  | table      | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
|  1 | PRIMARY      | user       | NULL       | index | NULL          | name | 402     | NULL |    1 |   100.00 | Using index     |
|  2 | UNION        | user       | NULL       | index | NULL          | name | 402     | NULL |    1 |   100.00 | Using index     |
| NULL | UNION RESULT | <union1,2> | NULL       | ALL   | NULL          | NULL | NULL    | NULL | NULL |     NULL | Using temporary |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)

对于union查询，会建立一个临时表，对应的id是null

select_type

SIMPLE

simple是简单查询，mysql认为比较简单的查询，不包含子查询和union查询都是simple，哪怕简单的join都是simple

mysql> explain select * from user;
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | user  | NULL       | index | NULL          | name | 402     | NULL |    1 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from user a left join user_info b on a.id=b.user_id;
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref       | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+-------------+
|  1 | SIMPLE      | a     | NULL       | index | NULL          | name    | 402     | NULL      |    1 |   100.00 | Using index |
|  1 | SIMPLE      | b     | NULL       | ref   | user_id       | user_id | 8       | test.a.id |    1 |   100.00 | NULL        |
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

PRIMARY

查询中若包含任何复杂的子部分，最外层查询则被标记为primary

mysql> explain select * from user where id = (select id from user_info where age=10);
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table     | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
|  1 | PRIMARY     | user      | NULL       | const | PRIMARY       | PRIMARY | 8       | const |    1 |   100.00 | NULL        |
|  2 | SUBQUERY    | user_info | NULL       | ALL   | NULL          | NULL    | NULL    | NULL  |    1 |   100.00 | Using where |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

mysql> explain select * from  user union all select * from user;
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
|  1 | PRIMARY     | user  | NULL       | index | NULL          | name | 402     | NULL |    1 |   100.00 | Using index |
|  2 | UNION       | user  | NULL       | index | NULL          | name | 402     | NULL |    1 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

SUBQUERY

在SELECT或WHERE列表中包含了子查询的语句

mysql> explain select (select id from user) from user;
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
|  1 | PRIMARY     | user  | NULL       | index | NULL          | name | 402     | NULL |    1 |   100.00 | Using index |
|  2 | SUBQUERY    | user  | NULL       | index | NULL          | name | 402     | NULL |    1 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

mysql> explain select * from user where id= (select user_id from user_info where age=10);
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table     | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
|  1 | PRIMARY     | user      | NULL       | const | PRIMARY       | PRIMARY | 8       | const |    1 |   100.00 | NULL        |
|  2 | SUBQUERY    | user_info | NULL       | ALL   | NULL          | NULL    | NULL    | NULL  |    1 |   100.00 | Using where |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

DERIVED

派生表的select(from子句的子查询)

mysql> explain select * from (select * from user union select * from user) c;
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type  | table      | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
|  1 | PRIMARY      | <derived2> | NULL       | ALL   | NULL          | NULL | NULL    | NULL |    4 |   100.00 | NULL            |
|  2 | DERIVED      | user       | NULL       | index | NULL          | name | 1023    | NULL |    1 |   100.00 | Using index     |
|  3 | UNION        | user       | NULL       | index | NULL          | name | 1023    | NULL |    1 |   100.00 | Using index     |
| NULL | UNION RESULT | <union2,3> | NULL       | ALL   | NULL          | NULL | NULL    | NULL | NULL |     NULL | Using temporary |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
4 rows in set, 1 warning (0.00 sec)

UNION

union中后面的select语句

mysql> explain select * from user union select * from user;
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type  | table      | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
|  1 | PRIMARY      | user       | NULL       | index | NULL          | name | 1023    | NULL |    1 |   100.00 | Using index     |
|  2 | UNION        | user       | NULL       | index | NULL          | name | 1023    | NULL |    1 |   100.00 | Using index     |
| NULL | UNION RESULT | <union1,2> | NULL       | ALL   | NULL          | NULL | NULL    | NULL | NULL |     NULL | Using temporary |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.02 sec)

DEPENDENT UNION

出如今union或union all语句中，可是这个查询要受到外部查询的影响

mysql> explain select * from user where id in(select id from user union select id from user);
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+
| id | select_type        | table      | partitions | type   | possible_keys | key     | key_len | ref  | rows | filtered | Extra                    |
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+
|  1 | PRIMARY            | user       | NULL       | index  | NULL          | name    | 1023    | NULL |    1 |   100.00 | Using where; Using index |
|  2 | DEPENDENT SUBQUERY | user       | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | func |    1 |   100.00 | Using index              |
|  3 | DEPENDENT UNION    | user       | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | func |    1 |   100.00 | Using index              |
| NULL | UNION RESULT       | <union2,3> | NULL       | ALL    | NULL          | NULL    | NULL    | NULL | NULL |     NULL | Using temporary          |
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+
4 rows in set, 1 warning (0.11 sec)

DEPENDENT SUBQUERY

同DEPENDENT UNION 差很少，包含子查询，且受到外部查询的影响

mysql> explain select * from user where id in(select id from user union select id from user);
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+
| id | select_type        | table      | partitions | type   | possible_keys | key     | key_len | ref  | rows | filtered | Extra                    |
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+
|  1 | PRIMARY            | user       | NULL       | index  | NULL          | name    | 1023    | NULL |    1 |   100.00 | Using where; Using index |
|  2 | DEPENDENT SUBQUERY | user       | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | func |    1 |   100.00 | Using index              |
|  3 | DEPENDENT UNION    | user       | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | func |    1 |   100.00 | Using index              |
| NULL | UNION RESULT       | <union2,3> | NULL       | ALL    | NULL          | NULL    | NULL    | NULL | NULL |     NULL | Using temporary          |
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+
4 rows in set, 1 warning (0.11 sec)

UNION RESULT

出如今union或union all中，表示的是一个结果集

mysql> explain select id from user union select id from user;
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type  | table      | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
|  1 | PRIMARY      | user       | NULL       | index | NULL          | name | 1023    | NULL |    1 |   100.00 | Using index     |
|  2 | UNION        | user       | NULL       | index | NULL          | name | 1023    | NULL |    1 |   100.00 | Using index     |
| NULL | UNION RESULT | <union1,2> | NULL       | ALL   | NULL          | NULL | NULL    | NULL | NULL |     NULL | Using temporary |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)

table

查询的表名，有时候显示的并非真正的表名

mysql> explain select * from (select * from user union select * from user) c;
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type  | table      | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
|  1 | PRIMARY      | <derived2> | NULL       | ALL   | NULL          | NULL | NULL    | NULL |    4 |   100.00 | NULL            |
|  2 | DERIVED      | user       | NULL       | index | NULL          | name | 1023    | NULL |    1 |   100.00 | Using index     |
|  3 | UNION        | user       | NULL       | index | NULL          | name | 1023    | NULL |    1 |   100.00 | Using index     |
| NULL | UNION RESULT | <union2,3> | NULL       | ALL   | NULL          | NULL | NULL    | NULL | NULL |     NULL | Using temporary |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
4 rows in set, 1 warning (0.00 sec)

好比id=1的table是derived2，说明它查的是一个派生表，其中derived[num],这个num就是第几步执行的结果，这里是2，说明就是id=2的那一步执行的结果。

partitions

查询匹配记录的分区。对于非分区表，值为NULL。
先建立一张分区表

CREATE TABLE users (
     id INT NOT NULL ,
     name varchar(100) NOT NULL 
)
PARTITION BY RANGE (id) (
     PARTITION p0 VALUES LESS THAN (10),
     PARTITION p1 VALUES LESS THAN (20),
     PARTITION p2 VALUES LESS THAN (30),
     PARTITION p3 VALUES LESS THAN MAXVALUE
);

经过id分了4个区，插入两条数据

mysql> select * from users;
+----+-------+
| id | name  |
+----+-------+
|  1 | Tom   |
| 11 | jerry |
+----+-------+
2 rows in set (0.00 sec)

mysql> explain select * from users where id=1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | users | p0         | ALL  | NULL          | NULL | NULL    | NULL |    1 |   100.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

id=1 分布在p0分区

mysql> explain select * from users where id=11;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | users | p1         | ALL  | NULL          | NULL | NULL    | NULL |    1 |   100.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

id=11 分布在p1分区

type

type是很是重要的指标，它表示咱们使用什么类型去查数据。下面由好到坏的介绍各个type类型

system

这种类型通常不会出现，官方解释：

The table has only one row (= system table). This is a special case of the const join type.

表只有一行记录，通常是系统表，是一种特殊的const类型

const

表最多有一个匹配行，由于只有一行，因此优化器的其他部分能够将此行中列的值视为常量。const表很是快，由于它们只读取一次。

mysql> explain select * from user where id=1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | user  | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)

const必定是用到primary key或者unique的索引时候才会出现。

mysql> explain select * from user where name="Tom";
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | user  | NULL       | ref  | name          | name | 1023    | const |    1 |   100.00 | Using index |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)

虽然经过name也查出一条数据，可是因为name是普通索引，因此不是const

eq_ref

除了system和const 它是最好的。通常出如今链接查询中，且链接的条件是主键索引或者惟一索引时，就会用到它。

mysql> explain select * from user a left join user_info b on a.id=b.user_id;
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
|  1 | SIMPLE      | a     | NULL       | index  | NULL          | name    | 1023    | NULL      |    1 |   100.00 | Using index |
|  1 | SIMPLE      | b     | NULL       | eq_ref | user_id       | user_id | 4       | test.a.id |    1 |   100.00 | NULL        |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

a.id是primary key
b.user_id是 unique key
先查a表，a表关联b表经过id和user_id来，由于id是主键索引，而user_id又是惟一索引，那么a表中的每条记录仅会关联b表的一条记录。因此b表的类型就是eq_ref。

ref

和eq_ref相比，不一样的就是关联表查询的字段不是惟一或者主键索引，就是匹配到了，还要继续匹配，可能有多条记录。

mysql> explain select b.* from user a left join user_info b on a.id=b.user_id;
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref       | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+-------------+
|  1 | SIMPLE      | a     | NULL       | index | NULL          | name    | 402     | NULL      |    1 |   100.00 | Using index |
|  1 | SIMPLE      | b     | NULL       | ref   | user_id       | user_id | 8       | test.a.id |    1 |   100.00 | NULL        |
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.01 sec)

a.id是primary key
b.user_id是普通的索引key
a可能关联b的多条记录，可是起码有索引，因此的b的type就是ref。

fulltext

全文索引的话，innodb不支持，随着各类搜索引擎的出现，通常出现须要全文索引的地方，都会用相似es擅长分词的存储。

ref_or_null

这种类型相似于ref，可是MySQL会额外搜索包含空值的行。

mysql> explain select * from user_info where user_id is null or user_id=1;
+----+-------------+-----------+------------+-------------+---------------+---------+---------+-------+------+----------+-----------------------+
| id | select_type | table     | partitions | type        | possible_keys | key     | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+-----------+------------+-------------+---------------+---------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | user_info | NULL       | ref_or_null | user_id       | user_id | 9       | const |  108 |   100.00 | Using index condition |
+----+-------------+-----------+------------+-------------+---------------+---------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

user_id是普通的索引key，且容许为null。

index_merge

索引合并

mysql> explain select * from user_info where id=1 or user_id=999;
+----+-------------+-----------+------------+-------------+-----------------+-----------------+---------+------+------+----------+-------------------------------------------+
| id | select_type | table     | partitions | type        | possible_keys   | key             | key_len | ref  | rows | filtered | Extra                                     |
+----+-------------+-----------+------------+-------------+-----------------+-----------------+---------+------+------+----------+-------------------------------------------+
|  1 | SIMPLE      | user_info | NULL       | index_merge | PRIMARY,user_id | PRIMARY,user_id | 8,9     | NULL |    2 |   100.00 | Using union(PRIMARY,user_id); Using where |
+----+-------------+-----------+------------+-------------+-----------------+-----------------+---------+------+------+----------+-------------------------------------------+
1 row in set, 1 warning (0.00 sec)

id是主键索引，user_id是普通索引，结果用了两个索引的合并

unique_subquery

用于where中的in形式子查询，子查询返回不重复值惟一值，能够彻底替换子查询，效率更高。 该类型替换了下面形式的IN子查询的ref。

explain select * from user_info where user_id in (select id from user where id>10);

子查询的id为primary key或者unique key。

index_subquery

相似于unique_subquery子查询，可是子查询返回的是非惟一索引。

explain select * from user_info where user_id in (select id from user where id>10);

子查询的id不是primary key和unique key。

range

索引范围扫描，通常条件使用了 >，<，between，in等运算符的查询。

mysql> explain select * from user where id>=1 and id <=100;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | user  | NULL       | range | PRIMARY       | PRIMARY | 8       | NULL |  100 |   100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

index

和全表扫差很少，可是只有索引树被扫描，这一般比全表扫快，由于索引文件一般比数据文件小。

mysql> explain select user_id from user_info;
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+
| id | select_type | table     | partitions | type  | possible_keys | key     | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | user_info | NULL       | index | NULL          | user_id | 9       | NULL | 100100 |   100.00 | Using index |
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

ALL

全表扫描

mysql> explain select * from user_info where age=1;
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table     | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | user_info | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 100100 |    10.00 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

age没有索引，引擎层作了全表扫。

possible_keys

查询可能使用到的索引。

mysql> explain select * from user_info where id>1 and user_id >2;
+----+-------------+-----------+------------+-------+-----------------+---------+---------+------+-------+----------+-------------+
| id | select_type | table     | partitions | type  | possible_keys   | key     | key_len | ref  | rows  | filtered | Extra       |
+----+-------------+-----------+------------+-------+-----------------+---------+---------+------+-------+----------+-------------+
|  1 | SIMPLE      | user_info | NULL       | range | PRIMARY,user_id | PRIMARY | 8       | NULL | 50050 |    50.00 | Using where |
+----+-------------+-----------+------------+-------+-----------------+---------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

id和user_id都有索引

key

执行器最终选择的索引

mysql> explain select * from user_info where id>1 and user_id >2;
+----+-------------+-----------+------------+-------+-----------------+---------+---------+------+-------+----------+-------------+
| id | select_type | table     | partitions | type  | possible_keys   | key     | key_len | ref  | rows  | filtered | Extra       |
+----+-------------+-----------+------------+-------+-----------------+---------+---------+------+-------+----------+-------------+
|  1 | SIMPLE      | user_info | NULL       | range | PRIMARY,user_id | PRIMARY | 8       | NULL | 50050 |    50.00 | Using where |
+----+-------------+-----------+------------+-------+-----------------+---------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

这里选择了主键索引。

key_len

使用的索引的长度，这里面的状况仍是挺复杂的，特别对于一些复合索引。
假设存在这样的表

CREATE TABLE `testlen` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `name1` varchar(10) not null default "",
  `name2` varchar(10),
  `num1` int(10) not null default 0,
  `num2` int(10),
   PRIMARY KEY (`id`),
   key(`name1`),
   key(`name2`),
   key(`num1`),
   key(`num2`)
   
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

1.当字段为定长时，如char，int这些，须要有一个字节来标记是否为空，not null的话就不须要。

mysql> explain select * from testlen where num1=1;
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | testlen | NULL       | ref  | num1          | num1 | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.03 sec)

mysql> explain select * from testlen where num2=1;
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | testlen | NULL       | ref  | num2          | num2 | 5       | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

2.当字段为变长的时候，如varchar这些，除了是否须要用一个字节来标记非not null的，还须要额外的两个字节标记长度

3.对于char、varchar这些，utf8编码的一个字符占用3个字节，utf8mb4一个字符占用4个字节

​

mysql> explain select * from testlen where name1='1';
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys | key   | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | testlen | NULL       | ref  | name1         | name1 | 42      | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)

对于utf8mb4编码的来讲，not null的varchar(10) 最终长度=10*4+2 = 42

mysql> explain select * from testlen where name2='1';
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys | key   | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | testlen | NULL       | ref  | name2         | name2 | 43      | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

对于utf8mb4编码的来讲，容许为null的varchar(10) 最终长度=10*4+2+1 = 43。

ref

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。

mysql> explain select * from user where id=1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | user  | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)

where 条件是id=1，那么对应的ref列就是常量const。

mysql> explain select * from user a left join user_info b on a.id=b.user_id;
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
|  1 | SIMPLE      | a     | NULL       | index  | NULL          | name    | 1023    | NULL      |    1 |   100.00 | Using index |
|  1 | SIMPLE      | b     | NULL       | eq_ref | user_id       | user_id | 4       | test.a.id |    1 |   100.00 | NULL        |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.02 sec)

user_info关联user表，关联的条件是user.id，故ref=test.a.id（test是库名）

rows

扫描出的行数，这个是个估算的值，并非真正的结果集

filtered

filtered表示返回结果的行数占需读取行数的百分比，filtered列的值依赖于统计信息。

extra

一些额外的信息，主要说明如何找到数据的。

using index

使用覆盖索引的时候就会出现，只查找索引列的值。

mysql> explain select user_id from user_info;
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table     | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | user_info | NULL       | index | NULL          | user_id | 4       | NULL |    2 |   100.00 | Using index |
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

using where

MYSQL服务器层将在存储引擎层返回行之后再应用WHERE过滤条件，通常发生在不能走索引扫描的状况下或者走索引扫描，可是有些查询列不在索引当中的状况下。

mysql> explain select * from user_info where user_id>10;
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table     | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | user_info | NULL       | ALL  | user_id       | NULL | NULL    | NULL | 100100 |    50.00 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.01 sec)

Using temporary

对于一些order by、group by可能会用到临时表。

mysql> explain select * from user a left join user_info b on a.id=b.user_id where a.id>=1 order by b.user_id;
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+----------------------------------------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref       | rows | filtered | Extra                                        |
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+----------------------------------------------+
|  1 | SIMPLE      | a     | NULL       | range | PRIMARY       | PRIMARY | 8       | NULL      | 5187 |   100.00 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | b     | NULL       | ref   | user_id       | user_id | 9       | test.a.id |    1 |   100.00 | Using where                                  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+----------------------------------------------+
2 rows in set, 1 warning (0.01 sec)

Using filesort

通常order by相关的没用到索引，就要文件排序。

mysql> explain select * from user_info order by age desc;
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+----------------+
| id | select_type | table     | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra          |
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+----------------+
|  1 | SIMPLE      | user_info | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 100100 |   100.00 | Using filesort |
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+----------------+

Using index condition

相比using where，Using index condition把where条件用到的索引放在引擎层过滤一下，没用到索引的列，在server层再过滤一遍。

mysql> explain select * from user a left join user_info b on a.id=b.user_id where b.user_id>1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+-------+----------+-----------------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref       | rows  | filtered | Extra                 |
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+-------+----------+-----------------------+
|  1 | SIMPLE      | a     | NULL       | index | PRIMARY       | name    | 402     | NULL      | 10375 |   100.00 | Using index           |
|  1 | SIMPLE      | b     | NULL       | ref   | user_id       | user_id | 9       | test.a.id |     1 |   100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+-------+----------+-----------------------+
2 rows in set, 1 warning (0.00 sec)

 

​