MySQL EXPLAIN输出列的详细解释

1. 简介

explain语句提供有关 mysql 如何执行语句的信息。

explain与select、delete、insert、replace和update语句一起使用。

mysql> explain select * from employees where emp_no = 10001;
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table     | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | extra |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | simple      | employees | null       | const | primary       | primary | 4       | const |    1 |   100.00 | null  |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

简单来讲，通过explain可以分析出sql语句走没走索引，走的是什么索引。

explain为select语句中使用的每个表返回一行信息，它按照 mysql 在处理语句时读取它们的顺序列出了输出中的表。

mysql 使用嵌套循环连接(nested-loop join algorithms)解析所有连接，这意味着 mysql 从第一个表中读取一行，然后在第二个表，第三个表中找到匹配的行，依此类推。处理完所有表后，mysql将通过表列表输出选定的列后回溯直到找到一个表，其中存在更多匹配的行。从该表中读取下一行，然后继续下一个表。

2.explain 输出列

• mysql版本 5.7.33
• windows10 64位

从上图看到 explain 的结果中，包括的表头id、select_type、table、partitions、type、possible_keys、key、key_len、ref、rows、filtered、extra，这些字段的意思我们来学习然后通过实例进行了解一下。

2.1 id

select 标识符，查询中 select 的顺序号。如果该行引用其他行的并集结果，则该值可以为null。在这种情况下，表列显示类似<unionm,n>的值，以指示该行引用 id 值为 m 和 n 的行的并集。

id 值分三种情况：

id 相同,执行顺序由上至下

mysql> explain (
    -> select * from employees emp
    -> left join dept_emp de on emp.emp_no = de.emp_no
    -> left join departments dept on dept.dept_no = de.dept_no
    -> where emp.emp_no = 10001);
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+-------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref                  | rows | filtered | extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+-------+
|  1 | simple      | emp   | null       | const  | primary       | primary | 4       | const                |    1 |   100.00 | null  |
|  1 | simple      | de    | null       | ref    | primary       | primary | 4       | const                |    1 |   100.00 | null  |
|  1 | simple      | dept  | null       | eq_ref | primary       | primary | 12      | employees.de.dept_no |    1 |   100.00 | null  |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+-------+
3 rows in set, 1 warning (0.03 sec)

id不相同，如果是子查询，id的序号会递增，id的值越大被执行的优先级越高

mysql> explain select * from employees emp
    -> where emp.emp_no not in ( select de.emp_no from dept_emp de 
    -> where de.dept_no not in ( select dept_no from departments where dept_name = 'development'));
+----+-------------+-------------+------------+-------+-------------------+-----------+---------+-------+--------+----------+--------------------------+
| id | select_type | table       | partitions | type  | possible_keys     | key       | key_len | ref   | rows   | filtered | extra                    |
+----+-------------+-------------+------------+-------+-------------------+-----------+---------+-------+--------+----------+--------------------------+
|  1 | primary     | emp         | null       | all   | null              | null      | null    | null  | 299468 |   100.00 | using where              |
|  2 | subquery    | de          | null       | index | primary           | dept_no   | 12      | null  | 308493 |   100.00 | using where; using index |
|  3 | subquery    | departments | null       | const | primary,dept_name | dept_name | 122     | const |      1 |   100.00 | using index              |
+----+-------------+-------------+------------+-------+-------------------+-----------+---------+-------+--------+----------+--------------------------+
3 rows in set, 1 warning (0.00 sec)

id相同和不相同都存在

如果id相同可以认为是一组，同一组id执行顺序由上至下，不同组之间，id值越大被执行的优先级越高。

mysql> explain select * from employees emp
    -> where emp.emp_no in ( select de.emp_no from dept_emp de 
    -> where de.dept_no in ( select dept_no from departments where dept_name like '%develop%'));
+----+--------------+-------------+------------+-------+-----------------+-----------+---------+-------------------------------+--------+----------+----------------------------------------------------+
| id | select_type  | table       | partitions | type  | possible_keys   | key       | key_len | ref                           | rows   | filtered | extra                                              |
+----+--------------+-------------+------------+-------+-----------------+-----------+---------+-------------------------------+--------+----------+----------------------------------------------------+
|  1 | simple       | <subquery2> | null       | all   | null            | null      | null    | null                          |   null |   100.00 | null                                               |
|  1 | simple       | emp         | null       | all   | primary         | null      | null    | null                          | 299468 |     0.00 | using where; using join buffer (block nested loop) |
|  2 | materialized | departments | null       | index | primary         | dept_name | 122     | null                          |      9 |    11.11 | using where; using index                           |
|  2 | materialized | de          | null       | ref   | primary,dept_no | dept_no   | 12      | employees.departments.dept_no |  38561 |   100.00 | using index                                        |
+----+--------------+-------------+------------+-------+-----------------+-----------+---------+-------------------------------+--------+----------+----------------------------------------------------+
4 rows in set, 1 warning (0.01 sec)

2.2 select_type

查询的类型，主要用来区别普通查询，联合查询，子查询等复杂查询。

包含simple、primary、union、dependent union、union result、subquery、dependent subquery、derived、materialized、uncacheable subquery、uncacheable union

simple

简单的select，不使用union或子查询。

mysql> explain select * from employees where emp_no=10001;
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table     | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | extra |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | simple      | employees | null       | const | primary       | primary | 4       | const |    1 |   100.00 | null  |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

primary

查询中若包含任何复杂的子部分，最外层的查询则被标记为primary

mysql> explain select * from employees emp
    -> where emp.emp_no in ( select max(emp_no) from dept_emp);
+----+--------------------+-------+------------+------+---------------+------+---------+------+--------+----------+------------------------------+
| id | select_type        | table | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | extra                        |
+----+--------------------+-------+------------+------+---------------+------+---------+------+--------+----------+------------------------------+
|  1 | primary            | emp   | null       | all  | null          | null | null    | null | 299468 |   100.00 | using where                  |
|  2 | dependent subquery | null  | null       | null | null          | null | null    | null |   null |     null | select tables optimized away |
+----+--------------------+-------+------------+------+---------------+------+---------+------+--------+----------+------------------------------+
2 rows in set, 1 warning (0.00 sec)

union

第二个或更靠后的 select 语句出现在 union 之后，则被标记为 union

mysql> explain (select emp_no,dept_no from dept_emp limit 10)
    -> union
    -> select emp_no,dept_no from dept_manager;
+----+--------------+--------------+------------+-------+---------------+---------+---------+------+--------+----------+-----------------+
| id | select_type  | table        | partitions | type  | possible_keys | key     | key_len | ref  | rows   | filtered | extra           |
+----+--------------+--------------+------------+-------+---------------+---------+---------+------+--------+----------+-----------------+
|  1 | primary      | dept_emp     | null       | index | null          | dept_no | 12      | null | 308493 |   100.00 | using index     |
|  2 | union        | dept_manager | null       | index | null          | dept_no | 12      | null |     24 |   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)

dependent union

与 union 相同，它出现在 union 或 union all语句中，但是此查询受外部查询的影响

| union result union_result result of a union.
| subquery none first select in subquery
| dependent subquery dependent (true) first select in subquery, dependent on outer query
| derived none derived table
| materialized materialized_from_subquery materialized subquery
| uncacheable subquery cacheable (false) a subquery for which the result cannot be cached and must be re-evaluated for each row of the outer query
| uncacheable union cacheable (false) the second or later select in a union that belongs to an uncacheable subquery (see uncacheable subquery)

总结

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