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

到此这篇关于MySQL EXPLAIN输出列的文章就介绍到这了,更多相关MySQL EXPLAIN输出列内容请搜索我们以前的文章或继续浏览下面的相关文章希望大家以后多多支持我们！