一文看懂如何分析MySQL Explain（2/3）
一文看懂如何分析MySQL Explain（2/3）
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发布于 2019-06-11 19:03:38
发布于 2019-06-11 19:03:38
接上文“一文看懂如何分析MySQL Explain（1/3）”
⑨ range：如果使用索引获取某些范围区间的记录，那么就可能使用到range访问方法，比如：
EXPLAIN EXTENDED select * from t_classes where id in (1,2,3); show WARNINGS;
+-------+-------------+-----------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+-------+-------------+-----------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | t_classes | range | PRIMARY | PRIMARY | 4 | NULL | 3 | 100.00 | Using where |
+-------+-------------+-----------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note | 1003 | select `test`.`t_classes`.`id` AS `id`,`test`.`t_classes`.`classes_name` AS `classes_name`,`test`.`t_classes`.`grade` AS `grade`,`test`.`t_classes`.`student_num` AS `student_num`,`test`.`t_classes`.`head_teacher_id` AS `head_teacher_id`,`test`.`t_classes`.`status` AS `status`,`test`.`t_classes`.`create_time` AS `create_time`,`test`.`t_classes`.`update_time` AS `update_time` from `test`.`t_classes` where (`test`.`t_classes`.`id` in (1,2,3)) |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+⑩ index：当我们可以使用索引覆盖，但需要扫描全部的索引记录时，该表的访问方法就是index，比如：
EXPLAIN EXTENDED select id_card from t_student where student_name = '张三';show WARNINGS;
+-------+-------------+-----------+-------+---------------+--------------------------+---------+------+------+----------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+-------+-------------+-----------+-------+---------------+--------------------------+---------+------+------+----------+--------------------------+
| 1 | SIMPLE | t_student | index | NULL | idx_classes_name_card_no | 1542 | NULL | 8 | 100.00 | Using where; Using index |
+-------+-------------+-----------+-------+---------------+--------------------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)
+-------+------+-------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------+
| Note | 1003 | select `test`.`t_student`.`id_card` AS `id_card` from `test`.`t_student` where (`test`.`t_student`.`student_name` = '张三') |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------+⑪ all：全表扫描，就不多赘述了
5 possible_keys列
在EXPLAIN语句输出的执行计划中，possible_keys列表示可能用到的索引有哪些，如：
EXPLAIN EXTENDED select id from t_student where classes_id = 1 ;SHOW WARNINGS;
+-------+-------------+-----------+------+-----------------------------------------+----------------+---------+-------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+-------+-------------+-----------+------+-----------------------------------------+----------------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | t_student | ref | idx_classes_id,idx_classes_name_card_no | idx_classes_id | 4 | const | 8 | 100.00 | Using index |
+-------+-------------+-----------+------+-----------------------------------------+----------------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
+-------+------+------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+------------------------------------------------------------------------------------------------------------+
| Note | 1003 | select `test`.`t_student`.`id` AS `id` from `test`.`t_student` where (`test`.`t_student`.`classes_id` = 1) |
+-------+------+------------------------------------------------------------------------------------------------------------+6 key列
在EXPLAIN语句输出的执行计划中，keys列表示经过查询优化器计算使用不同索引成本后决定使用的索引名，如上例
7 key_len列
key_len列表示当优化器决定使用某个索引执行查询时，该索引记录的最大长度，它是由这三个部分构成的：
① 对于使用固定长度类型的索引列来说，它实际占用的存储空间的最大长度就是该固定值，对于指定字符集的变长类型的索引列来说，比如某个索引列的类型是VARCHAR(255)，使用的字符集是utf8（MySQL utf8字符集占用3个字节长度），那么该列实际占用的最大存储空间就是255 × 3 + 2= 767个字节。
② 如果该索引列可以存储NULL值，则key_len比不可以存储NULL值时多1个字节。
③ 对于变长字段来说，都会有2个字节的空间来存储该变长列的实际长度。
例1：classes_id字段是int类型，非空，占用了四个字节，长度是4
EXPLAIN EXTENDED select id from t_student where classes_id = 1; SHOW WARNINGS;
+----+-------------+-----------+------+-----------------------------------------+----------------+---------+-------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------+-----------------------------------------+----------------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | t_student | ref | idx_classes_id,idx_classes_name_card_no | idx_classes_id | 4 | const | 8 | 100.00 | Using index |
+----+-------------+-----------+------+-----------------------------------------+----------------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
+-------+------+------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+------------------------------------------------------------------------------------------------------------+
| Note | 1003 | select `test`.`t_student`.`id` AS `id` from `test`.`t_student` where (`test`.`t_student`.`classes_id` = 1) |
+-------+------+------------------------------------------------------------------------------------------------------------+例2：classes_id字段是int类型，字段改为可为空，占用了四个字节，可为空加一个字节，长度是5
+--------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| classes_id | int(11) | YES | MUL | NULL | |
| student_name | varchar(255) | NO | | NULL | |
| age | int(11) | NO | | NULL | |
| id_card | varchar(255) | NO | UNI | NULL | |
| address | varchar(255) | NO | | NULL | |
| no | int(11) | NO | | NULL | |
| status | tinyint(4) | NO | | NULL | |
+--------------+--------------+------+-----+---------+----------------+EXPLAIN EXTENDED select id from t_student where classes_id = 1; SHOW WARNINGS;
+----+-------------+-----------+------+-----------------------------------------+----------------+---------+-------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------+-----------------------------------------+----------------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | t_student | ref | idx_classes_id,idx_classes_name_card_no | idx_classes_id | 5 | const | 8 | 100.00 | Using index |
+----+-------------+-----------+------+-----------------------------------------+----------------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
+-------+------+------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+------------------------------------------------------------------------------------------------------------+
| Note | 1003 | select `test`.`t_student`.`id` AS `id` from `test`.`t_student` where (`test`.`t_student`.`classes_id` = 1) |
+-------+------+------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)例3: student_name字段是varchar(255)，255 * 3 + 2= 767
EXPLAIN EXTENDED select id from t_student where student_name = ''; SHOW WARNINGS;
+----+-------------+-----------+------+------------------+------------------+---------+-------+------+----------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------+------------------+------------------+---------+-------+------+----------+--------------------------+
| 1 | SIMPLE | t_student | ref | idx_student_name | idx_student_name | 767 | const | 1 | 100.00 | Using where; Using index |
+----+-------------+-----------+------+------------------+------------------+---------+-------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)
+-------+------+---------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+---------------------------------------------------------------------------------------------------------------+
| Note | 1003 | select `test`.`t_student`.`id` AS `id` from `test`.`t_student` where (`test`.`t_student`.`student_name` = '') |
+-------+------+---------------------------------------------------------------------------------------------------------------+8 ref列
当使用索引列等值匹配的条件去执行查询时，也就是在访问方法是const、eq_ref、ref、ref_or_null、unique_subquery、index_subquery其中之一时，ref列展示的就是与索引列作等值匹配的条件，如常数或某个列。
例1: 过滤条件是常数
EXPLAIN EXTENDED select id from t_student where student_name = '';
+----+-------------+-----------+------+------------------+------------------+---------+-------+------+----------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------+------------------+------------------+---------+-------+------+----------+--------------------------+
| 1 | SIMPLE | t_student | ref | idx_student_name | idx_student_name | 767 | const | 1 | 100.00 | Using where; Using index |
+----+-------------+-----------+------+------------------+------------------+---------+-------+------+----------+--------------------------+例2:过滤条件是字段，下例过滤条件是test数据库的t_student表的classes_id字段
EXPLAIN EXTENDED select t_student.id from t_student inner join t_classes on t_classes.id = t_student.classes_id;
+----+-------------+-----------+--------+-----------------------------------------+----------------+---------+---------------------------+------+----------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+--------+-----------------------------------------+----------------+---------+---------------------------+------+----------+--------------------------+
| 1 | SIMPLE | t_student | index | idx_classes_id,idx_classes_name_card_no | idx_classes_id | 5 | NULL | 8 | 100.00 | Using where; Using index |
| 1 | SIMPLE | t_classes | eq_ref | PRIMARY | PRIMARY | 4 | test.t_student.classes_id | 1 | 100.00 | Using index |
+----+-------------+-----------+--------+-----------------------------------------+----------------+---------+---------------------------+------+----------+--------------------------+9 rows列
该列是查询优化器预计的扫描记录数。
例1:使用idx_classes_id索引，代表扫描的索引记录数
EXPLAIN EXTENDED select t_student.id from t_student ;
+----+-------------+-----------+-------+---------------+----------------+---------+------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+-------+---------------+----------------+---------+------+------+----------+-------------+
| 1 | SIMPLE | t_student | index | NULL | idx_classes_id | 5 | NULL | 8 | 100.00 | Using index |
+----+-------------+-----------+-------+---------------+----------------+---------+------+------+----------+-------------+例2: 使用全表扫描，代表表记录数
EXPLAIN EXTENDED select * from t_classes where student_num > 56;
+----+-------------+-----------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | t_classes | ALL | NULL | NULL | NULL | NULL | 20 | 100.00 | Using where |
+----+-------------+-----------+------+---------------+------+---------+------+------+----------+-------------+10 filtered列
filtered列在单表查询时没什么意义，但是在连接查询时意义重大，凭借该列和rows列可以粗略估算出当前连接的查询成本，MySQL连接查询采用的是嵌套循环连接算法，驱动表会被访问一次，被驱动表可能会被访问多次，所以MySQL连接查询的成功 = 单次驱动表查询成本 + 单次被驱动表查询成本 * 查询次数（MySQL执行计划估算出的驱动表查询结果集记录数，即rows列的值）
EXPLAIN EXTENDED select t_subject.id from t_subject inner join t_classes on t_classes.id = t_subject.classes_id; SHOW WARNINGS;
+----+-------------+-----------+-------+----------------+----------------+---------+-------------------+-------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+-------+----------------+----------------+---------+-------------------+-------+----------+-------------+
| 1 | SIMPLE | t_classes | index | PRIMARY | idx_grade | 4 | NULL | 20 | 100.00 | Using index |
| 1 | SIMPLE | t_subject | ref | idx_classes_id | idx_classes_id | 4 | test.t_classes.id | 43718 | 100.00 | Using index |
+----+-------------+-----------+-------+----------------+----------------+---------+-------------------+-------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note | 1003 | select `test`.`t_subject`.`id` AS `id` from `test`.`t_subject` join `test`.`t_classes` where (`test`.`t_subject`.`classes_id` = `test`.`t_classes`.`id`) |
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------+如上例，t_subject是驱动表，t_classes是被驱动表，MySQL是按照执行计划中的顺序逐条执行（id序号，如果相同则从上往下执行），首先从t_classes表中查询出所有的记录，因为可以使用覆盖索引，但是要扫描所有的记录行，所以table为t_classes的执行计划行的type是index，预计读取到20行记录（t_classes表实际有20行记录），预计符合条件的记录百分比是100.00%（filtered字段的值），把从驱动表（t_classes）中获取到的记录数和被驱动表（t_subject）中的记录做匹配，最终预计读取到43718条记录
11 Extra列
Extra列是用来说明一些额外信息的，我们可以通过这些额外信息来更准确的理解MySQL到底将如何执行给定的查询语句。MySQL提供的额外信息有好几十个，我们就不一个一个介绍了，在这只介绍常见的一些额外信息说明
① No tables used： 当查询语句没有FROM子句时将会提示该额外信息，比如：
EXPLAIN EXTENDED select now();
+----+-------------+-------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+----------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+----------------+② Impossible WHERE：查询语句的WHERE子句永远为FALSE时将会提示该额外信息，如：
EXPLAIN EXTENDED select t_subject.id from t_subject where 1 = 0;
+----+-------------+-------+------+---------------+------+---------+------+------+----------+------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+------------------+③ Using index：当我们的查询列表以及搜索条件中只包含属于某个索引的列，也就是在可以使用索引覆盖的情况下，在Extra列将会提示该额外信息，不会再执行回表操作。如以下这个查询中只需要用到idx_name_grade索引而不需要回表操作：
EXPLAIN EXTENDED select classes_name from t_classes where grade = 55;
+----+-------------+-----------+------+--------------------------+----------------+---------+-------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------+--------------------------+----------------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | t_classes | ref | idx_grade,idx_name_grade | idx_name_grade | 4 | const | 1 | 100.00 | Using index |
+----+-------------+-----------+------+--------------------------+----------------+---------+-------+------+----------+-------------+④ Using index condition：查找使用了索引，但是需要回表查询数据
EXPLAIN EXTENDED select * from t_student where classes_id > 10;
+----+-------------+-----------+-------+-----------------------------------------+----------------+---------+------+------+----------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+-------+-----------------------------------------+----------------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | t_student | range | idx_classes_id,idx_classes_name_card_no | idx_classes_id | 5 | NULL | 1 | 100.00 | Using index condition |
+----+-------------+-----------+-------+-----------------------------------------+----------------+---------+------+------+----------+-----------------------+⑤ Using where：当我们使用全表扫描来执行对某个表的查询，并且该语句的WHERE子句中有针对该表的搜索条件时，在Extra列中会提示上述额外信息，如：
EXPLAIN EXTENDED select * from t_student where address = '中国';
+----+-------------+-----------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | t_student | ALL | NULL | NULL | NULL | NULL | 8 | 100.00 | Using where |
+----+-------------+-----------+------+---------------+------+---------+------+------+----------+-------------+⑥ Using join buffer：在连接查询执行过程中，当被驱动表不能有效的利用索引加快访问速度，MySQL一般会为其分配一块名叫join buffer的内存块来加快查询速度，如下：
EXPLAIN EXTENDED select * from t_student INNER JOIN t_student_copy1 on t_student.`no` = t_student_copy1.`no` and t_student.classes_id = t_student_copy1.classes_id;show WARNINGS;
+------+-------------+-----------------+------+-----------------------------------------+------+---------+------+------+----------+-------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+------+-------------+-----------------+------+-----------------------------------------+------+---------+------+------+----------+-------------------------------------------------+
| 1 | SIMPLE | t_student_copy1 | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | |
| 1 | SIMPLE | t_student | ALL | idx_classes_id,idx_classes_name_card_no | NULL | NULL | NULL | 8 | 75.00 | Using where; Using join buffer (flat, BNL join) |
+------+-------------+-----------------+------+-----------------------------------------+------+---------+------+------+----------+-------------------------------------------------+如上例，先查询出t_student_copy1表中的所有数据（只有一条），然后再循环匹配表t_student中的数据，但是因为不能使用索引，只好退而求其次，使用join buffer减少对表t_student的访问次数，提高性能
⑦ Not exists：当我们使用外连接时，如果WHERE子句中包含要求被驱动表的某个列是NULL值的搜索条件，且那个列是非NULL的，那么在该表的执行计划的Extra列就会提示Not exists额外信息，比如以下SQL，查询出所有未关联学生的班级信息：
EXTENDED select * from t_classes left join t_student on t_student.classes_id = t_classes.id where t_student.id is null;
+----+-------------+-----------+------+-----------------------------------------+------+---------+------+------+----------+-------------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------+-----------------------------------------+------+---------+------+------+----------+-------------------------------------------------------------+
| 1 | SIMPLE | t_classes | ALL | NULL | NULL | NULL | NULL | 20 | 100.00 | |
| 1 | SIMPLE | t_student | ALL | idx_classes_id,idx_classes_name_card_no | NULL | NULL | NULL | 8 | 75.00 | Using where; Not exists; Using join buffer (flat, BNL join) |
+----+-------------+-----------+------+-----------------------------------------+------+---------+------+------+----------+-------------------------------------------------------------+上述查询中t_classes表是驱动表，t_student表是被驱动表，t_student.id列是非空的，而WHERE子句中又包含t_student.id IS NULL的搜索条件，这意味着必定是驱动表的记录在被驱动表中找不到匹配ON子句条件的记录才会把该驱动表的记录加入到最终的结果集，所以对于某条驱动表中的记录来说，如果能在被驱动表中找到1条符合ON子句条件的记录，那么该驱动表的记录就不会被加入到最终的结果集，也就是说我们没有必要到被驱动表中找到全部符合ON子句条件的记录，这样可以稍微节省一点性能。
⑧ Using filesort：如果根据索引列进行排序（order by 索引列）是可以用到索引的，SQL查询引擎会先根据索引列进行排序，然后获取对应记录的主键id执行回表操作，如果排序字段用不到索引则只能在内存中或磁盘中进行排序操作，MySQL把这种在内存或者磁盘上进行排序的方式统称为文件排序（英文名：filesort），如果某个查询需要使用文件排序的方式执行查询，就会在执行计划的Extra列中显示Using filesort，如：
例1: 未使用文件排序
EXPLAIN EXTENDED select id from t_score order by score;
+----+-------------+---------+-------+---------------+-----------+---------+------+--------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+-------+---------------+-----------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | t_score | index | NULL | idx_score | 4 | NULL | 201062 | 100.00 | Using index |
+----+-------------+---------+-------+---------------+-----------+---------+------+--------+----------+-------------+EXPLAIN EXTENDED select * from t_score order by id;
+----+-------------+---------+-------+---------------+---------+---------+------+--------+----------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+-------+---------------+---------+---------+------+--------+----------+-------+
| 1 | SIMPLE | t_score | index | NULL | PRIMARY | 4 | NULL | 201062 | 100.00 | |
+----+-------------+---------+-------+---------------+---------+---------+------+--------+----------+-------+例2: 使用文件排序
EXPLAIN EXTENDED select * from t_score order by subject_id;
+----+-------------+---------+------+---------------+------+---------+------+--------+----------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------+---------------+------+---------+------+--------+----------+----------------+
| 1 | SIMPLE | t_score | ALL | NULL | NULL | NULL | NULL | 201062 | 100.00 | Using filesort |
+----+-------------+---------+------+---------------+------+---------+------+--------+----------+----------------+注：Using filesort比较耗费性能，如果有可能尽量优化成使用索引排序
⑨ Using temporary：许多查询的执行过程中，MySQL会借助临时表来完成一些功能，比如去重、排序之类的，比如我们在执行许多包含DISTINCT、GROUP BY、UNION等子句的查询过程中，如果不能有效利用索引来完成查询，MySQL很有可能寻求通过建立内部的临时表来执行查询。如果查询中使用到了内部的临时表，在执行计划的Extra列将会显示Using temporary提示，如：
EXPLAIN EXTENDED select distinct subject_name from t_subject;
+----+-------------+-----------+------+---------------+------+---------+------+--------+----------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------+---------------+------+---------+------+--------+----------+-----------------+
| 1 | SIMPLE | t_subject | ALL | NULL | NULL | NULL | NULL | 786925 | 100.00 | Using temporary |
+----+-------------+-----------+------+---------------+------+---------+------+--------+----------+-----------------+接“一文看懂如何分析MySQL Explain（3/3）”
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