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。
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)
