EXPLAIN語句提供有關 MySQL 如何執行語句的信息。

EXPLAIN與SELECT、DELETE、INSERT、REPLACE和UPDATE語句一起使用。

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)

簡單來講，通過EXPLAIN可以分析出SQL語句走沒走索引，走的是什么索引。

EXPLAIN為SELECT語句中使用的每個表返回一行信息，它按照 MySQL 在處理語句時讀取它們的順序列出了輸出中的表。

MySQL 使用嵌套循環連接(Nested-Loop Join Algorithms)解析所有連接，這意味著 MySQL 從第一個表中讀取一行，然后在第二個表，第三個表中找到匹配的行，依此類推。處理完所有表后，MySQL將通過表列表輸出選定的列后回溯直到找到一個表，其中存在更多匹配的行。從該表中讀取下一行，然后繼續下一個表。

從上圖看到 EXPLAIN 的結果中，包括的表頭id、select_type、table、partitions、type、possible_keys、key、key_len、ref、rows、filtered、Extra，這些字段的意思我們來學習然后通過實例進行了解一下。

2.1 id

SELECT 標識符，查詢中 SELECT 的順序號。如果該行引用其他行的并集結果，則該值可以為NULL。在這種情況下，表列顯示類似<unionM,N>的值，以指示該行引用 id 值為 M 和 N 的行的并集。

id 值分三種情況：

id 相同,執行順序由上至下

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，如果是子查詢，id的序號會遞增，id的值越大被執行的優先級越高

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相同可以認為是一組，同一組id執行順序由上至下，不同組之間，id值越大被執行的優先級越高。

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

查詢的類型，主要用來區別普通查詢，聯合查詢，子查詢等復雜查詢。

包含SIMPLE、PRIMARY、UNION、DEPENDENT UNION、UNION RESULT、SUBQUERY、DEPENDENT SUBQUERY、DERIVED、MATERIALIZED、UNCACHEABLE SUBQUERY、UNCACHEABLE UNION

SIMPLE

簡單的SELECT，不使用UNION或子查詢。

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

查詢中若包含任何復雜的子部分，最外層的查詢則被標記為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

第二個或更靠后的 SELECT 語句出現在 UNION 之后，則被標記為 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

與 UNION 相同，它出現在 UNION 或 UNION ALL語句中，但是此查詢受外部查詢的影響

| 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)

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