mysql 索引
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EXPLAIN 关键字可以用于获取所修饰语句执行计划的相关信息，在 MySQL 8.0 中，EXPLAIN 支持大多数常见的语句，如 SELECT 、DELETE 、INSERT 、REPLACE、和 UPDATE 语句。示例如下：
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```sql
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-- 查询工资大于100000的雇员所在部门的编号
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EXPLAIN SELECT * FROM dept_emp WHERE emp_no IN (SELECT emp_no FROM salaries WHERE salary > 100000) LIMIT 100;
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mysql> EXPLAIN SELECT * FROM employees;
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+----+-------------+-----------+------+---------------+------+---------+------+--------+-------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+-----------+------+---------------+------+---------+------+--------+-------+
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| 1 | SIMPLE | employees | ALL | NULL | NULL | NULL | NULL | 299379 | NULL |
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+----+-------------+-----------+------+---------------+------+---------+------+--------+-------+
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```
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注：本篇文章所有的测试数据来均源于 MySQL 官方提供的 [Employees Sample Database](https://dev.mysql.com/doc/employee/en/)，其数据库结构如下：
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<div align="center"> <img src="https://github.com/heibaiying/Full-Stack-Notes/blob/master/pictures/employees-schema.png"/> </div>
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以下对 EXPLAIN 输出结果中各个字段的含义及其可能的取值范围做详细的介绍：
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## 1. id
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id 字段为行标识符，同时也表示该行语句执行的优先级，值越大则优先级越高。如果某行语句引用了其他行结果集的并集，则该值可以为 NULL。示例如下：
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```sql
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mysql> EXPLAIN SELECT * FROM dept_emp WHERE emp_no IN (SELECT emp_no FROM salaries WHERE salary > 100000) LIMIT 100;
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+----+-------------+----------+------+---------------+---------+---------+---------------------------+--------+-----------------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+----------+------+---------------+---------+---------+---------------------------+--------+-----------------------------------+
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| 1 | SIMPLE | dept_emp | ALL | PRIMARY | NULL | NULL | NULL | 331570 | NULL |
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| 1 | SIMPLE | salaries | ref | PRIMARY | PRIMARY | 4 | employees.dept_emp.emp_no | 4 | Using where; FirstMatch(dept_emp) |
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+----+-------------+----------+------+---------------+---------+---------+---------------------------+--------+-----------------------------------+
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```
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> 注：本篇文章测试数据来源于 MySQL 官方提供的 [Employees Sample Database](https://dev.mysql.com/doc/employee/en/)
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### 1. id
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每个 SELECT 语句都会对应输出结果中的一行数据，id 列是该行数据的编号，如果在语句当中没有子查询或者联合查询，则每行数据的编号都是 1 。如果涉及到 FROM 子查询 或者 UNION 查询，则编号通常会按照其在原始语句中的位置排序。示例如下：
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```sql
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-- 该FROM字句只是用于演示
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mysql> EXPLAIN SELECT COUNT(1) FROM (SELECT emp_no FROM salaries) AS s;
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+----+-------------+------------+-------+
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| id | select_type | table | type |
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+----+-------------+------------+-------+
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| 1 | PRIMARY | <derived2> | ALL |
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| 2 | DERIVED | salaries | index |
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+----+-------------+------------+-------+ ......
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-- 该FROM字句只是用于演示，并没有任何实际意义
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mysql> EXPLAIN SELECT COUNT(1) FROM (SELECT emp_no FROM salaries) AS t;
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+----+-------------+------------+-------+---------------+---------+---------+------+---------+-------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+------------+-------+---------------+---------+---------+------+---------+-------------+
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| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 2757635 | NULL |
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| 2 | DERIVED | salaries | index | NULL | PRIMARY | 7 | NULL | 2757635 | Using index |
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+----+-------------+------------+-------+---------------+---------+---------+------+---------+-------------+
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```
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```sql
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-- 查询工资大于500000或部门编号等于d007的所有雇员的编号
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mysql> EXPLAIN SELECT emp_no FROM salaries WHERE salary>500000 UNION ALL SELECT emp_no FROM dept_emp WHERE dept_no = "d007";
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+----+--------------+------------+------+
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| id | select_type | table | type |
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+----+--------------+------------+------+
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| 1 | PRIMARY | salaries | ALL |
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| 2 | UNION | dept_emp | ref |
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| NULL | UNION RESULT | <union1,2> | ALL |
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+----+--------------+------------+------+ ......
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mysql> EXPLAIN SELECT emp_no FROM salaries WHERE salary>500000 UNION ALL SELECT emp_no FROM dept_emp WHERE dept_no = "d007";
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+----+--------------+------------+------+---------------+---------+---------+-------+---------+--------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+--------------+------------+------+---------------+---------+---------+-------+---------+--------------------------+
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| 1 | PRIMARY | salaries | ALL | NULL | NULL | NULL | NULL | 2837161 | Using where |
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| 2 | UNION | dept_emp | ref | dept_no | dept_no | 4 | const | 91566 | Using where; Using index |
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| NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
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+----+--------------+------------+------+---------------+---------+---------+-------+---------+--------------------------+
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```
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### 2. select_type
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## 2. select_type
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### 3. table
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+ **SIMPLE**：不包含子查询或者 UNION 操作的查询；
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+ **PRIMARY**：查询中如果包含任何子查询，那么最外层的查询则被标记为 PRIMARY ；
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+ **SUBQUERY**：子查询中第一个 SELECT ；
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+ **DEPENDENT SUBQUERY**：子查询中的第一个 SELECT，取决于外部查询；
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+ **UNION**：UNION 操作的第二个或者之后的查询；
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+ **DEPENDENT UNION**：UNION 操作的第二个或者之后的查询,取决于外部查询；
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+ **UNION RESULT**：UNION 产生的结果集；
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+ **DERIVED**：出现在 FROM 字句中的子查询。
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这里常用的类型在上面的示例中都出现过，下面演示查询类型为 SUBQUERY 的情况，示例如下：
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```sql
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-- 根据员工编号查询员工姓名及其工资总和
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mysql> explain select first_name,(select sum(salary) from salaries where emp_no = 10001) from employees where emp_no = 10001;
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+----+-------------+-----------+-------+---------------+---------+---------+-------+------+-------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+-----------+-------+---------------+---------+---------+-------+------+-------+
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| 1 | PRIMARY | employees | const | PRIMARY | PRIMARY | 4 | const | 1 | NULL |
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| 2 | SUBQUERY | salaries | ref | PRIMARY | PRIMARY | 4 | const | 17 | NULL |
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+----+-------------+-----------+-------+---------------+---------+---------+-------+------+-------+
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```
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## 3. table
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- **<union*M*,*N*>**：输出结果中编号为 M 的行与编号为 N 的行的结果集的并集。
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- **<derived*N*>**：输出结果中编号为 N 的行的结果集，使用 derived 修饰表示这是一个派生结果集，如来自于 FROM 子句中的子查询 。
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- **<derived*N*>**：输出结果中编号为 N 的行的结果集，使用 derived 修饰表示这是一个派生结果集，如来自于 FROM 子句中的查询 。
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- **<subquery*N*>**：输出结果中编号为 N 的行的结果集，使用 subquery 修饰表示其来源于一个物化子查询。
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### 4. type
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## 4. type
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这个字段表示 MySQL 如何查找表中的行。常见参数按照查找性能由低到高，排序如下：
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这个字段非常重要，它表示 MySQL 使用何种方式来查找目标数据，不同查找方式会导致不同的性能开销，常见查找方式及其性能表现按照由高到低的顺序排序如下：
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+ ALL：全表扫描；
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+ index：按照索引顺序而不是行顺序进行全表扫描；
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+ range：范围扫描；
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+ ref：
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**1. system**：这是 const 类型的一个特例，只会出现在待查询的表只有一行数据的情况下。
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### 5. possible_keys
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**2. const**：出现在主键或唯一索引与常量值进行比较的场景下，这种情况下查询性能是最优的。
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```sql
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mysql> explain select * from employees where emp_no = 10008;
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+----+-------------+-----------+-------+---------------+---------+---------+-------+------+-------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+-----------+-------+---------------+---------+---------+-------+------+-------+
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| 1 | SIMPLE | employees | const | PRIMARY | PRIMARY | 4 | const | 1 | NULL |
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+----+-------------+-----------+-------+---------------+---------+---------+-------+------+-------+
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```
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**3. eq_ref**：在连接查询中，当连接使用索引的所有部分并且索引是 PRIMARY KEY 或 UNIQUE NOT NULL 索引时使用它。
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```sql
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-- 这里员工部门关系表 dept_no 的联合主键为 emp_no + dept_no ，即员工编号+部门标号
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mysql> explain select * from employees e,dept_emp d where e.emp_no = d.emp_no and dept_no = "d005";
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+----+-------------+-------+--------+-----------------+---------+---------+--------------------+------+-----------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+-------+--------+-----------------+---------+---------+--------------------+------+-----------------------+
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| 1 | SIMPLE | d | ref | PRIMARY,dept_no | dept_no | 4 | const | 1 | Using index condition |
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| 1 | SIMPLE | e | eq_ref | PRIMARY | PRIMARY | 4 | employees.d.emp_no | 1 | NULL |
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+----+-------------+-------+--------+-----------------+---------+---------+--------------------+------+-----------------------+
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```
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**4. ref**：通常在连接查询中，如果连接仅使用了前缀索引或用于连接的条件不是 PRIMARY KEY 或 UNIQUE 索引时使用它。
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```sql
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-- 这里仅使用了前缀索引emp_no，所以其类型为 ref , 而不是 eq_ref
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mysql> explain select * from employees e,dept_emp d where e.emp_no = d.emp_no;
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+----+-------------+-------+------+---------------+---------+---------+--------------------+--------+-------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+-------+------+---------------+---------+---------+--------------------+--------+-------+
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| 1 | SIMPLE | e | ALL | PRIMARY | NULL | NULL | NULL | 299379 | NULL |
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| 1 | SIMPLE | d | ref | PRIMARY | PRIMARY | 4 | employees.e.emp_no | 1 | NULL |
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+----+-------------+-------+------+---------------+---------+---------+--------------------+--------+-------+
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```
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**5. ref_or_null**：类似于 ref 类型的查询，但是附加了对 NULL 值列的查询。这里给出示例语句：
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```sql
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SELECT * FROM ref_table WHERE key_column=expr OR key_column IS NULL;
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```
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**6. index_merge**：该联接类型表示使用了索引进行合并优化，示例如下：
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```
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mysql> explain select * from dept_emp where dept_no = "d004" and emp_no < 10020;
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+----+-------------+----------+-------------+-----------------+-----------------+---------+------+------+-----------------------------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+----------+-------------+-----------------+-----------------+---------+------+------+-----------------------------------------------+
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| 1 | SIMPLE | dept_emp | index_merge | PRIMARY,dept_no | dept_no,PRIMARY | 8,4 | NULL | 1 | Using intersect(dept_no,PRIMARY); Using where |
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+----+-------------+----------+-------------+-----------------+-----------------+---------+------+------+-----------------------------------------------+
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```
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**7. range**：使用索引进行范围扫描，常见于 between、> 、< 这样的查询条件。
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```sql
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mysql> explain select * from employees where emp_no > 10000;
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+----+-------------+-----------+-------+---------------+---------+---------+------+--------+-------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+-----------+-------+---------------+---------+---------+------+--------+-------------+
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| 1 | SIMPLE | employees | range | PRIMARY | PRIMARY | 4 | NULL | 149689 | Using where |
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+----+-------------+-----------+-------+---------------+---------+---------+------+--------+-------------+
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```
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**8. index**：索引连接类型与 ALL 相同，只是扫描的是索引树，通常出现在索引是该查询的覆盖索引的情况：
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```sql
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mysql> explain select emp_no from employees;
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+----+-------------+-----------+-------+---------------+---------+---------+------+--------+-------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+-----------+-------+---------------+---------+---------+------+--------+-------------+
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| 1 | SIMPLE | employees | index | NULL | PRIMARY | 4 | NULL | 299379 | Using index |
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+----+-------------+-----------+-------+---------------+---------+---------+------+--------+-------------+
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```
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**9. ALL**：全表扫描，效率最差的查找方式。
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```sql
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mysql> explain select * from employees where first_name ="Bezalel";
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+----+-------------+-----------+------+---------------+------+---------+------+--------+-------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+-----------+------+---------------+------+---------+------+--------+-------------+
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| 1 | SIMPLE | employees | ALL | NULL | NULL | NULL | NULL | 299379 | Using where |
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+----+-------------+-----------+------+---------------+------+---------+------+--------+-------------+
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```
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## 5. possible_keys
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表示在执行过程中可能会用到哪些索引来进行优化。
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### 6. key
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## 6. key
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表示在执行过程中实际用到的用于优化的索引。
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### 7. key_len
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## 7. key_len
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表示使用到的索引的字节数。
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### 8. ref
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## 8. ref
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显示哪些列或常量与 key 列中指定的索引进行比较。
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### 9. rows
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## 9. rows
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MySQL 为了找到目标行而读取的所有行的数量，这是一个估算的值。
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### 10. Extra
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## 10. Extra
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用于显示额外的信息，常用
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用于显示额外的信息，常见参数如下：
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### 11. partitions
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+ **Distinct**：优化 distinct 操作，再找到第一匹配的元组后即停止找同样值的动作；
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+ **Not exists**：使用 not exists 来优化查询；
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+ **Using filesort**：使用额外操作进行排序，通常会出现在 order by 或 group by 查询中；
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+ **Using index**：使用了覆盖索引进行查询；
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+ **Using temporary**：使用临时表来处理查询，常见于排序、子查询和分组查询；
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+ **Using where**：使用 WHERE 条件来过滤数据；
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+ **select tables optimized away**：直接通过索引来获取数据，不需要访问表。
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显示查询结果集中记录所在的分区。如果目标表不是分区表，则值为 null 。
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## 11. partitions
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显示查询结果集中记录所在的分区。如果目标表不是分区表，则值为 null 。
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## 参考资料
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MySQL官方文档：https://dev.mysql.com/doc/refman/8.0/en/explain-output.html
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