Mysql 性能优化神器Explain详解
2024-06-05 02:17:17
文章目录
- Explain的作用
- 博文背景
- Explain 使用方法
- Explain之ID说明
- 1. ID值相同
- 2.ID值不同
- 3.ID值相同，ID值不同共存
- Explain的select_type详解
- SIMPLE 简单查询
- PRIMARY 主查询
- DERIVED 衍生
- UNION 联合
- SUBQUERY 子查询
- UNION RESULT 联合结果
- Explain的table详解
- Explain的type域详解
- 1.system
- 2.const
- 3.eq_ref
- 4.ref
- 5.range
- 6.index
- 7.all
- Explain之possible_keys域详解
- Explain之key域详解
- Explain之ref域详解
- Explain之rows域详解
- Explain之Extra域详解
- 1. Using filesort（文件排序）
- 2. Using temporary（使用临时表）
- 3. Using index.(使用索引)
- 4. Using Where
- 5. Using join buffer 使用了连接缓存
- 6. impossible where
- 7. select tables optimized away
- 8. distinct
- 总结
Explain的作用
使用Explain 关键字可以模拟优化器执行SQL查询语句，从而知道MySQL是如何处理我们的SQL语句的，就可以根据explain获取到的执行计划分析我们SQL语句或是表结构的性能瓶颈，从而调整表结构的索引或是SQL语句的优化。
博文背景
下文中所有的操作都基于这两张表，student，class表，表结构和索引如下
mysql> desc student;
+---------+------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------+------------+------+-----+---------+----------------+
| id | bigint | NO | PRI | NULL | auto_increment |
| name | varchar(9) | YES | | NULL | |
| age | int | YES | | NULL | |
| classId | bigint | NO | | NULL | |
+---------+------------+------+-----+---------+----------------+
4 rows in set (0.02 sec)mysql> show index from student;
+---------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| student | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | | YES | NULL |
| student | 0 | idx_name | 1 | name | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL |
| student | 1 | idx_classId | 1 | classId | A | 3 | NULL | NULL | | BTREE | | | YES | NULL |
+---------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
3 rows in set (0.04 sec)mysql>
mysql> desc class;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | bigint | NO | PRI | NULL | auto_increment |
| name | varchar(32) | YES | | NULL | |
+-------+-------------+------+-----+---------+----------------+
2 rows in set (0.03 sec)mysql> show index from class;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| class | 0 | PRIMARY | 1 | id | A | 5 | NULL | NULL | | BTREE | | | YES | NULL |
| class | 1 | idx_name | 1 | name | A | 5 | NULL | NULL | YES | BTREE | | | YES | NULL |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
2 rows in set (0.05 sec)mysql>
Explain 使用方法
Explain + SQL语句，就可以看到相应的执行计划，如下所示。
mysql> explain select * from student;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | student | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | NULL |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set (0.03 sec)mysql>
由上我们知道，Explain的结果中有如下几个信息项，下面我将一一来看看每一个选项的作用。
| 信息项 | 说明 |
| ID | 查询的序列号 |
| select_type | 查询类型 |
| table | 操作的表 |
| partitions | 操作的分区 |
| possible_keys | 可能使用到的索引名称 |
| key | 实际使用的索引名称 |
| key_len | 索引的长度 |
| ref | 被引用的索引列 |
| rows | 扫描的行数 |
| filtered | 过滤 |
| Extra | 其他额外信息 |
Explain之ID说明
explain的ID指的是查询的序列号，由一组数字组成，表示查询中执行select字句或者操作表的顺序。它包含以下三种情况：
1. ID值相同
ID值相同，按顺序由上到下执行操作
mysql> explain select * from student left join class on student.classId = class.id;
+----+-------------+---------+------------+--------+---------------+---------+---------+----------------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+--------+---------------+---------+---------+----------------------+------+----------+-------+
| 1 | SIMPLE | student | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | NULL |
| 1 | SIMPLE | class | NULL | eq_ref | PRIMARY | PRIMARY | 8 | news.student.classId | 1 | 100.00 | NULL |
+----+-------------+---------+------------+--------+---------------+---------+---------+----------------------+------+----------+-------+
2 rows in set (0.03 sec)mysql>
2.ID值不同
ID值越大，执行的优先级越高。
mysql> explain select classId,name from student union all select * from class where id = 2;
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | PRIMARY | student | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | NULL |
| 2 | UNION | class | NULL | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.00 | NULL |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
如上所示两条记录的id值递增，表示mysql先操作表class，然后再操作表student，由上到下顺序执行。
3.ID值相同，ID值不同共存
ID值越大的先执行，ID值相同的按照顺序由上到下执行。
mysql> explain select * from student where classId in (select id from class) union all (select * from student where id = 2);
+----+-------------+---------+------------+--------+---------------+---------+---------+----------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+--------+---------------+---------+---------+----------------------+------+----------+-------------+
| 1 | PRIMARY | student | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | NULL |
| 1 | PRIMARY | class | NULL | eq_ref | PRIMARY | PRIMARY | 8 | news.student.classId | 1 | 100.00 | Using index |
| 3 | UNION | student | NULL | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.00 | NULL |
+----+-------------+---------+------------+--------+---------------+---------+---------+----------------------+------+----------+-------------+
3 rows in set (0.03 sec)
Explain的select_type详解
查询的类型，主要用于区分普通查询、联合查询、子查询等复杂的查询。
select_type的取值范围有
| 取值 | 说明 |
| SIMPLE | 简单的select查询，查询中不包含子查询或者UNION |
| PRIMARY | 查询中包含任何复杂的子查询，最外层查询则被标记为PRIMARY |
| SUBQUERY | 在SELECT或者WHERE列表中包含子查询 |
| DERIVED | 在From列表中包含的子查询被标记为DERIVED(衍生)，MYSQL会递归执行这些子查询，将结果放置到临时表中 |
| UNION | 若第二个select出现在union后，则被标记为UNION |
| UNION RESULT | 从Union表获取结果的select |
SIMPLE 简单查询
mysql> explain select * from student where classId in (select name from class);
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------------------------------------------------------+
| 1 | SIMPLE | student | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | NULL |
| 1 | SIMPLE | class | NULL | ALL | NULL | NULL | NULL | NULL | 5 | 20.00 | Using where; FirstMatch(student); Using join buffer (hash join) |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------------------------------------------------------+
2 rows in set (0.03 sec)
PRIMARY 主查询
DERIVED 衍生
UNION 联合
mysql> explain SELECT * FROM (select * from student where classId in (select id from class) union all (select * from student where id = 2)) A WHERE A.ID = 3;
+----+-------------+------------+------------+--------+---------------+-------------+---------+----------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+--------+---------------+-------------+---------+----------------------+------+----------+-------------+
| 1 | PRIMARY | <derived2> | NULL | ref | <auto_key0> | <auto_key0> | 8 | const | 1 | 100.00 | NULL |
| 2 | DERIVED | student | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | NULL |
| 2 | DERIVED | class | NULL | eq_ref | PRIMARY | PRIMARY | 8 | news.student.classId | 1 | 100.00 | Using index |
| 4 | UNION | student | NULL | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.00 | NULL |
+----+-------------+------------+------------+--------+---------------+-------------+---------+----------------------+------+----------+-------------+
4 rows in set (0.03 sec)
SUBQUERY 子查询
mysql> explain select * from student A where classid = (select id from class where name = '一1班');
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | PRIMARY | A | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 25.00 | Using where |
| 2 | SUBQUERY | class | NULL | ALL | NULL | NULL | NULL | NULL | 5 | 20.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
2 rows in set (0.04 sec)
UNION RESULT 联合结果
Explain的table详解
操作的是哪张表，有时候会在表后面加序号如dirived2
mysql> explain SELECT * FROM (select * from student where classId in (select id from class) union all (select * from student where id = 2)) A WHERE A.ID = 3;
+----+-------------+------------+------------+--------+---------------+-------------+---------+----------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+--------+---------------+-------------+---------+----------------------+------+----------+-------------+
| 1 | PRIMARY | <derived2> | NULL | ref | <auto_key0> | <auto_key0> | 8 | const | 1 | 100.00 | NULL |
| 2 | DERIVED | student | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | NULL |
| 2 | DERIVED | class | NULL | eq_ref | PRIMARY | PRIMARY | 8 | news.student.classId | 1 | 100.00 | Using index |
| 4 | UNION | student | NULL | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.00 | NULL |
+----+-------------+------------+------------+--------+---------------+-------------+---------+----------------------+------+----------+-------------+
4 rows in set (0.03 sec)
Explain的type域详解
显示查询使用了何种类型，所有的取值范围如下表所示
| ALL | INDEX | RANGE | REF | EQ_REF | CONST,SYSTEM | NULL |
从最好到最差依次是：SYSTEM>CONST>EQ_REF>REF>RANGE>INDEX>ALL
1.system
表中只有一条记录（等于系统表）,这是const类型的特例，可以忽略不计
2.const
表示通过索引，一次就找到了符合条件的记录，const 用于比较primary Key 或者unique索引。因为只匹配一行数据，所以很快。
mysql> explain select * from student where id = 1;
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | student | NULL | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.00 | NULL |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set (0.03 sec)
3.eq_ref
唯一性索引扫描，对于每个索引键表中只有一条记录与之匹配，常见于主键或唯一索引。如下所示，classId在class表中只有一条记录
mysql> select * from student;
+----+--------+-----+---------+
| id | name | age | classId |
+----+--------+-----+---------+
| 1 | 张三 | 12 | 1 |
| 2 | 李四 | 14 | 2 |
| 3 | 张三风 | 16 | 1 |
| 4 | 李四海 | 19 | 2 |
| 5 | 王老吉 | 20 | 3 |
+----+--------+-----+---------+
5 rows in set (0.02 sec)mysql> select * from class;
+----+--------+
| id | name |
+----+--------+
| 1 | 一一班 |
| 2 | 一2班 |
| 3 | 一3班 |
| 4 | 一4班 |
| 5 | 一5班 |
| 6 | 一6班 |
+----+--------+
6 rows in set (0.04 sec)mysql> explain select * from student where exists (select 1 from class where id = student.classId);
+----+--------------------+---------+------------+--------+---------------+---------+---------+----------------------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+---------+------------+--------+---------------+---------+---------+----------------------+------+----------+--------------------------+
| 1 | PRIMARY | student | NULL | ALL | NULL | NULL | NULL | NULL | 5 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | class | NULL | eq_ref | PRIMARY | PRIMARY | 8 | news.student.classId | 1 | 100.00 | Using where; Using index |
+----+--------------------+---------+------------+--------+---------------+---------+---------+----------------------+------+----------+--------------------------+
2 rows in set (0.04 sec)4.ref
非唯一索引扫描，返回匹配某个单独值的所有行。
mysql> explain select * from student where classId = 1;
+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+
| 1 | SIMPLE | student | NULL | ref | idx_classId_age | idx_classId_age | 4 | const | 2 | 100.00 | NULL |
+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+
1 row in set (0.02 sec)5.range
通过索引扫描的是一个范围
mysql> explain select * from student where classId >= (select id from class where name = '一一班' limit 1);
+----+-------------+---------+------------+-------+---------------+-------------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+-------------+---------+-------+------+----------+-------------+
| 1 | PRIMARY | student | NULL | range | idx_classId | idx_classId | 8 | NULL | 4 | 100.00 | Using where |
| 2 | SUBQUERY | class | NULL | ref | idx_name | idx_name | 131 | const | 1 | 100.00 | Using index |
+----+-------------+---------+------------+-------+---------------+-------------+---------+-------+------+----------+-------------+
2 rows in set (0.04 sec)
6.index
全索引扫描,index与ALL的区别是index类型只遍历索引树、这通常会比ALL快，因为索引文件比数据文件小。
mysql> explain select name from student;
+----+-------------+---------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| 1 | SIMPLE | student | NULL | index | NULL | idx_name | 39 | NULL | 4 | 100.00 | Using index |
+----+-------------+---------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
1 row in set (0.03 sec)7.all
全表扫描
mysql> explain select * from student;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | student | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 25.00 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set (0.04 sec)
Explain之possible_keys域详解
显示可能应用在这张表中的索引，一个或者多个、查询涉及到的字段上若存在索引，则该索引被列出，但是并不一定被查询实际所使用到。
Explain之key域详解
实际使用的索引，如果为NULL，则没有使用索引。（查询中使用了覆盖索引，则该索引和查询中的select 字段重叠）
Explain之ref域详解
显示索引的那一列被使用了，如果可能的话是一个常数(const)
mysql> explain select * from student where classId = 1 and age = 12;
+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------------+------+----------+-------+
| 1 | SIMPLE | student | NULL | ref | idx_classId_age | idx_classId_age | 9 | const,const | 1 | 100.00 | NULL |
+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------------+------+----------+-------+
1 row in set (0.03 sec)mysql>
例如，student 表有复合索引，上述查询语句中全匹配，classId 和age ，使用到了索引的两个列，所以ref中出现两个const。
Explain之rows域详解
根据表统计信息及索引选用情况、大致估算出找到所需记录需要读取的行数。
Explain之Extra域详解
包含不适合在其他列中展示但十分重要的额外信息
1. Using filesort（文件排序）
说明mysql会对数据使用一个外部的索引排序，而不是按照表内的索引顺序进行读取。 mysql中无法利用索引完成的排序称之为“文件排序”,这种情况应当极力避免。
mysql> explain select * from student where classId >= 0 order by age;
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+---------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+---------------------------------------+
| 1 | SIMPLE | student | NULL | range | idx_classId | idx_classId | 8 | NULL | 4 | 100.00 | Using index condition; Using filesort |
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+------+----------+---------------------------------------+
1 row in set (0.07 sec)2. Using temporary（使用临时表）
使用了临时表保存中间的结果，MYSQL在对查询结果进行排序是使用临时表、常见于排序order by 和分组查询group by，这种情况也应当极力避免，比如使用覆盖索引。
mysql> explain select distinct classId,age,name from student;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 1 | SIMPLE | student | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | Using temporary |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-----------------+
1 row in set (0.03 sec)
3. Using index.(使用索引)
表示相应的select 操作中使用了覆盖索引(Covering Index)，避免了访问数据表的数据行，效率不错！
如果同时出现了Using where,表明索引用来执行索引键值的查找；
如果没有出现Using Where，表明索引用来读取数据而非执行查找。
mysql> explain select classId,age from student where classId > 1 ;
+----+-------------+---------+------------+-------+-----------------+-----------------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+-----------------+-----------------+---------+------+------+----------+--------------------------+
| 1 | SIMPLE | student | NULL | range | idx_classId_age | idx_classId_age | 4 | NULL | 2 | 100.00 | Using where; Using index |
+----+-------------+---------+------------+-------+-----------------+-----------------+---------+------+------+----------+--------------------------+
1 row in set (0.03 sec)
4. Using Where
表示出现了where过滤，如上所示。
5. Using join buffer 使用了连接缓存
mysql> explain select distinct classId,student.age,student.name from student left join class on student.name = class.name;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+--------------------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+--------------------------------------------------------------+
| 1 | SIMPLE | student | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | Using temporary |
| 1 | SIMPLE | class | NULL | ALL | NULL | NULL | NULL | NULL | 6 | 100.00 | Using where; Distinct; Using join buffer (Block Nested Loop) |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+--------------------------------------------------------------+
2 rows in set (0.02 sec)mysql>
6. impossible where
where字句的值总是false，不能用来获取任何元素
mysql> explain select * from student where false ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
1 row in set (0.03 sec)
7. select tables optimized away
mysql> explain select max(classId),min(classId) from student ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
1 row in set (0.04 sec)8. distinct
优化distinct操作，在找到第一匹配的元祖后立即停止找同样值的动作。
示例操作见(Using join buffer)
总结
至此，Explain的运用全部介绍完毕，希望此文能够帮助到大家更好的使用mysql的SQL语言性能优化神器Explain。
需要重点记住的有如下几点：
Explain 的type从最好到最差依次是：SYSTEM>CONST>EQ_REF>REF>RANGE>INDEX>ALL,所以应当尽量避免全表扫描，并按照该顺序进行SQL语句的优化。
需要重点关注Key，它指的是实际使用的索引名称。
Extra域中需要重点关注是否出现“Using filesort” 和“Using temporary”，这两者应当尽量避免，会严重影响性能。
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