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1. 簡介

EXPLAIN語句提供有關(guān) MySQL 如何執(zhí)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(huán)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(xù)下一個表。

2.EXPLAIN 輸出列

MySQL版本 5.7.33
Windows10 64位

從上圖看到 EXPLAIN 的結(jié)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(xué)習(xí)然后通過實(shí)例進(jìn)行了解一下。

2.1 id

SELECT 標(biāo)識符，查詢中 SELECT 的順序號。如果該行引用其他行的并集結(jié)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(zhí)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(zhí)行的優(yōu)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(rèn)為是一組，同一組id執(zhí)行順序由上至下，不同組之間，id值越大被執(zhí)行的優(yōu)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(qū)別普通查詢，聯(lián)合查詢，子查詢等復(fù)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(fù)雜的子部分，最外層的查詢則被標(biāo)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(xiàn)在 UNION 之后，則被標(biāo)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(xiàn)在 UNION 或 UNION ALL語句中，但是此查詢受外部查詢的影響

| UNION RESULTunion_resultResult of a UNION.
| SUBQUERYNoneFirst SELECT in subquery
| DEPENDENT SUBQUERYdependent (true)First SELECT in subquery, dependent on outer query
| DERIVEDNoneDerived table
| MATERIALIZEDmaterialized_from_subqueryMaterialized subquery
| UNCACHEABLE SUBQUERYcacheable (false)A subquery for which the result cannot be cached and must be re-evaluated for each row of the outer query
| UNCACHEABLE UNIONcacheable (false)The second or later select in a UNION that belongs to an uncacheable subquery (see UNCACHEABLE SUBQUERY)

總結(jié)

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