一、explain是什么？
1、 定义
EXPLAIN是mysql中的一个命令，可以模拟优化器执行SQL语句并返回执行计划。通过执行计划，我们可以分析查询语句或表结构的性能瓶颈，从而进行SQL优化。
2、用法
mysql> explain select * from employee; +----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+ | 1 | SIMPLE | employee | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | +----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec)
通过上面的执行计划我们可以了解：
- 表的执行顺序
- 数据读取操作的操作类型
- 哪些索引可以使用
- 哪些索引被实际使用
- 每张表有多少行被优化器查询
看到这个表，也许你会两眼一抹黑，这咋看呀？接着往下看，你一定可以从执行计划中轻松得出以上结果。
二、名词段解释
名词
1. id
定义：select查询的序列号，包含一组数字，表示查询中执行select子句或操作表的顺序。
存在以下三种情况：
- id相同，执行顺序由上至下
mysql> desc select e.*,p.* from employee e,performance p where p.user_id = e.id; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------------------+ | 1 | SIMPLE | e | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | | 1 | SIMPLE | p | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50.00 | Using where; Using join buffer (hash join) | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------------------+ 2 rows in set, 1 warning (0.00 sec)
- id不同，如果是子查询，id序号对递增，id值越大优先级越高，越先执行
mysql> explain select e.* -> from employee e -> where id = ( select user_id -> from performance -> where performance.score > 80 and performance.year = 2020 and performance.quarter = 2); +----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | PRIMARY | e | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where | | 2 | SUBQUERY | performance | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50.00 | Using where | +----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+ 2 rows in set, 1 warning (0.01 sec)
- id相同而不同（不同的：id越大越先执行；相同的：由上至下按顺序执行）
mysql> EXPLAIN SELECT -> e.*, d.depart_name -> FROM employee e,department d -> WHERE -> e.depart_id = d.id -> AND e.id = ( -> SELECT user_id -> WHERE -> performance.score > 80 -> AND performance. YEAR = 2020 -> AND performance. QUARTER = 2 -> ); +----+-------------+-------------+------------+--------+---------------+---------+---------+----------------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------------+------------+--------+---------------+---------+---------+----------------+------+----------+-------------+ | 1 | PRIMARY | e | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where | | 1 | PRIMARY | d | NULL | eq_ref | PRIMARY | PRIMARY | 4 | db.e.depart_id | 1 | 100.00 | NULL | | 2 | SUBQUERY | performance | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50.00 | Using where | +----+-------------+-------------+------------+--------+---------------+---------+---------+----------------+------+----------+-------------+ 3 rows in set, 1 warning (0.00 sec)
2、select_type
定义：查询的类型，主要用于区别普通查询、联合查询和子查询等复合查询。
- SIMPLE：简单的select查询，查询中不包含子查询和联合查询（union）
mysql> explain select * from employee; +----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+ | 1 | SIMPLE | employee | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | +----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec)
- PRIMARY：若查询中包含任何的子查询，最外层查询则标记为
PRIMARY - SUBQUERY：在select或where列表中包含的子查询
mysql> explain select e.* -> from employee e -> where id = ( select user_id -> from performance -> where performance.score > 80 and performance.year = 2020 and performance.quarter = 2); +----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | PRIMARY | e | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where | | 2 | SUBQUERY | performance | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50.00 | Using where | +----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+ 2 rows in set, 1 warning (0.01 sec)
- DERIVED：在FROM列表中包含的子查询被标记为（DERIVED）,Mysql会递归执行这些子查询，把结果放在临时表里。
mysql> EXPLAIN select employee.* FROM (SELECT distinct department.id FROM department) s1,employee WHERE s1.id = employee.depart_id; +----+-------------+------------+------------+-------+---------------+-------------+---------+-----------------------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+-------+---------------+-------------+---------+-----------------------+------+----------+-------------+ | 1 | PRIMARY | employee | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where | | 1 | PRIMARY | <derived2> | NULL | ref | <auto_key0> | <auto_key0> | 4 | db.employee.depart_id | 2 | 100.00 | Using index | | 2 | DERIVED | department | NULL | index | PRIMARY | PRIMARY | 4 | NULL | 4 | 100.00 | Using index | +----+-------------+------------+------------+-------+---------------+-------------+---------+-----------------------+------+----------+-------------+ 3 rows in set, 1 warning (0.00 sec)
- UNION：如果第二个SELECT出现在UNION之后，则标记为UNION;如果UNION包含在FROM子句的子查询中，外层SELECT被标记为DERIVED
- UNION RESULT：从UNION表获取结果的SELECT
mysql> explain select * from employee union select * from employee_asu; +----+--------------+--------------+------------+------+---------------+------+---------+------+------+----------+-----------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+--------------+--------------+------------+------+---------------+------+---------+------+------+----------+-----------------+ | 1 | PRIMARY | employee | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | | 2 | UNION | employee_asu | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | | NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary | +----+--------------+--------------+------------+------+---------------+------+---------+------+------+----------+-----------------+ 3 rows in set, 1 warning (0.00 sec) mysql> explain select * from (select * from employee union select * from employee_asu) s; +----+--------------+--------------+------------+------+---------------+------+---------+------+------+----------+-----------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+--------------+--------------+------------+------+---------------+------+---------+------+------+----------+-----------------+ | 1 | PRIMARY | <derived2> | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | NULL | | 2 | DERIVED | employee | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | | 3 | UNION | employee_asu | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | | NULL | UNION RESULT | <union2,3> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary | +----+--------------+--------------+------------+------+---------------+------+---------+------+------+----------+-----------------+ 4 rows in set, 1 warning (0.00 sec)
3、table
定义：顾名思义，表名。
4、partitions（分区）
定义：显示表分区的分区名
mysql> explain select * from tk; +----+-------------+-------+------------+------+---------------+------+---------+--- | id | select_type | table | partitions | type | possible_keys | key | key_len | re +----+-------------+-------+------------+------+---------------+------+---------+--- | 1 | SIMPLE | tk | p0,p1,p2 | ALL | NULL | NULL | NULL | NU +----+-------------+-------+------------+------+---------------+------+---------+--- 1 row in set, 1 warning (0.00 sec)
5、type
定义：type显示的是访问类型，是较为重要的一个指标，结果值从最好到最坏依次为system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > all，常见的有system > const > eq_ref > ref > range > index > all
- system：表只有一行记录（等于系统表），这是const类型的特例，平时不会出现，可忽略不计。
mysql> EXPLAIN select * FROM (SELECT distinct department.id FROM department where id=2) s1; +----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+ | 1 | PRIMARY | <derived2> | NULL | system | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | | 2 | DERIVED | department | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | Using index | +----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)
- const：表示通过索引一次就找到了，const用于比较primary key或者unique索引，只匹配一行数据，速度很快。
mysql> explain SELECT distinct department.id FROM department where id=2; +----+-------------+------------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+ | 1 | SIMPLE | department | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | Using index | +----+-------------+------------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)
- eq_ref：唯一性索引扫描，对于每个索引键，表中只有一条数据与之匹配，常见于主键或唯一索引扫描。
mysql> explain SELECT * from employee,department where employee.depart_id = department.id ; +----+-------------+------------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+ | 1 | SIMPLE | employee | NULL | ALL | NULL | NULL | NULL | NULL | 7 | 100.00 | Using where | | 1 | SIMPLE | department | NULL | eq_ref | PRIMARY | PRIMARY | 4 | db.employee.depart_id | 1 | 100.00 | NULL | +----+-------------+------------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)
- ref：非唯一性索引扫描，返回匹配某个单独值的所有行
mysql> show index from employee; #查看表索引 +----------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression | +----------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | employee | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | | YES | NULL | | employee | 1 | idx_employee_username | 1 | username | A | 0 | NULL | NULL | YES | BTREE | | | YES | NULL | +----------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ 2 rows in set (0.00 sec) mysql> explain select * from employee where username = "john"; +----+-------------+----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+ | 1 | SIMPLE | employee | NULL | ref | idx_employee_username | idx_employee_username | 83 | const | 1 | 100.00 | NULL | +----+-------------+----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.01 sec)
- range：只检索给定范围的行，使用一个索引来选择行。key列显示使用了那个索引，一般是where语句中出现了between、<、>或in等查询。
mysql> explain select * from employee where id between 1 and 3; +----+-------------+----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | 1 | SIMPLE | employee | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 3 | 100.00 | Using where | +----+-------------+----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)
- index：full index scan,全索引树扫描，通常比all快，因为索引文件比数据文件小很多。
mysql> explain select id from department; +----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | 1 | SIMPLE | department | NULL | index | NULL | PRIMARY | 4 | NULL | 4 | 100.00 | Using index | +----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)
- all：全表扫描，当数据量达到百万级别，性能会下降明显，需要建立索引；当然小数据量的全表扫描是没有问题的。
mysql> explain select * from department; +----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+ | 1 | SIMPLE | department | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | NULL | +----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec)
6、possible_keys
定义：显示可能应用在这张表中的索引，一个或多个（查询涉及到的字段若存在索引，则该索引将被列出，但不一定被查询实际使用）
mysql> explain select id,username,age,salary from employee where username='a'; +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ | 1 | SIMPLE | employee | NULL | ref | idx_username_age_salary,idx_username | idx_username_age_salary | 83 | const | 1 | 100.00 | Using index | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)
补充说明：从执行计划关键字possible_keys中可以看出，sql语句执行可能用到idx_username和idx_username_age_salary两个索引。如果为null，就是没有可能使用的索引。
7、key
定义：实际使用的索引。如果为null，则没有使用索引。
- 常规举例
mysql> explain select username,age,salary from employee where username = 'a' group by age,salary; +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ | 1 | SIMPLE | employee | NULL | ref | idx_username_age_salary,idx_username | idx_username_age_salary | 83 | const | 1 | 100.00 | Using index | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)
补充说明：possible_keys，可能使用的所以为idx_username,idx_username_age_salary; key,实际使用的索引为idx_username_age_salary
- 覆盖索引
看到覆盖索引也许你两眼一抹黑，别急，先记住这么一个名词，继续往下看。
mysql> explain select username,age,salary from employee; +----+-------------+----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+ | 1 | SIMPLE | employee | NULL | index | NULL | idx_username_age_salary | 93 | NULL | 7 | 100.00 | Using index | +----+-------------+----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)
补充说明：如果查询中使用了覆盖索引，则该索引只会出现在key中，不会出现在possible_keys中。
8、key_len
定义：表示索引中使用的字节数，可通过该列计算查询中使用的索引长度，在不损失精度的情况下，key_len越短越好。换句话说，key_len显示的值为索引字段的最大可能长度，即key_len是通过表定义计算而得，不是通过表内检索出来的，不过具体怎么算出来的没有研究。
9、ref
定义：显示索引哪一列别使用了，如果可能的话，是一个常数。指明哪些列或常量被用于查找索引列上的值。
mysql> EXPLAIN SELECT employee.username, employee.age, employee.salary FROM employee, department WHERE employee.id = department.id; +----+-------------+------------+------------+--------+---------------+---------+---------+------------------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+--------+---------------+---------+---------+------------------+------+----------+-------------+ | 1 | SIMPLE | department | NULL | index | PRIMARY | PRIMARY | 4 | NULL | 4 | 100.00 | Using index | | 1 | SIMPLE | employee | NULL | eq_ref | PRIMARY | PRIMARY | 4 | db.department.id | 1 | 100.00 | NULL | +----+-------------+------------+------------+--------+---------------+---------+---------+------------------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)
补充说明：ref对呀的值为db.department.id,表示数据库db中的department表中的id列。
mysql> EXPLAIN SELECT username,age,salary FROM employee where username = 'john'; +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ | 1 | SIMPLE | employee | NULL | ref | idx_username_age_salary,idx_username | idx_username_age_salary | 83 | const | 1 | 100.00 | Using index | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)
补充说明：username对应的值为john,所以是常量const
10、rows
定义：根据表的统计信息及索引使用情况，大致估算出找到所需的记录需要读取的行数。
11、filtered
定义：这个字段表示存储引擎返回的数据在server层过滤后，剩下多少满足查询的记录数量的比例，注意是百分比，不是具体记录数。值越大越好。
12、Extra
定义：包含不适合在其它列中显示但十分重要的的额外信息
- Using filesort：说明Mysql使用一个外部索引排序，而不是按照表内的索引进行排序（Mysql中无法使用索引完成的排序操作称为“文件排序”）。
mysql> EXPLAIN SELECT username,age,salary FROM employee where username = 'john' order by id; +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-----------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-----------------------------+ | 1 | SIMPLE | employee | NULL | ref | idx_username_age_salary,idx_username | idx_username_age_salary | 83 | const | 1 | 100.00 | Using index; Using filesort | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-----------------------------+ 1 row in set, 1 warning (0.00 sec)
补充说明：虽然使用了idx_username_age_salary索引，但是排序规则不是按照这个索引来的，而是索引文件外排序。因此sql语句的排序规则尽量按照索引的字段来排序，避免文件外排序，降低性能。
- using temporary：表示使用了临时表保存了中间结果，Mysql在对查询结果排序时使用临时表，常见于排序order by和分组gruop by。
mysql> explain select age from employee where username in ('john','asd') group by age; +----+-------------+----------+------------+-------+------------------------------------------------------------+-------------------------+---------+------+------+----------+-------------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+-------+------------------------------------------------------------+-------------------------+---------+------+------+----------+-------------------------------------------+ | 1 | SIMPLE | employee | NULL | range | idx_username_age_salary,idx_username,idx_username_departid | idx_username_age_salary | 83 | NULL | 2 | 100.00 | Using where; Using index; Using temporary | +----+-------------+----------+------------+-------+------------------------------------------------------------+-------------------------+---------+------+------+----------+-------------------------------------------+ 1 row in set, 1 warning (0.00 sec)
补充说明：产生临时表很消耗性能
- using index：表示相应的select操作中使用了覆盖索引，避免访问了数据行，效率不错。如果同时出现了using where，表示在索引中查找；如果没有，表明是从索引中读取数据，而非查找。
覆盖索引/索引覆盖(Covering index)：select数据列只需要从索引中就可以全部获取，不必读取数据行，换句话说查询列要被所建的索引覆盖。
mysql> explain select username,age,salary from employee where username ='john' order by age,salary; +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ | 1 | SIMPLE | employee | NULL | ref | idx_username_age_salary,idx_username | idx_username_age_salary | 83 | const | 1 | 100.00 | Using index | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)
注意：要想使用覆盖索引，一定要从select列表中获取自己想要的列，不可使用select *;如果使用所有列做索引会导致索引过于庞大。
- using where：sql语句中使用了where过滤
- using join buffer：使用了连接缓存
- impossible where：where子句中总是false，不可能查到任何数据
- select table optimized away：在没有group by的情况下，基于索引优化MIN/MAX操作或者对于MyISAM存储优化count(*)操作，不必等到执行阶段在进行计算，执行计划完成的阶段就完成优化。
- distinct：优化distinct操作，再找到第一匹配的元组后就停止找同样值的操作。
