MySQL优化之EXPLAIN命令解析

转自：https://juejin.cn/post/7073761727850119199

EXPLAIN：查看SQL语句的执行计划

EXPLAIN命令可以帮助我们深入了解MySQL基于开销的优化器，还可以获得很多可能被优化器考虑到的访问策略的细节，以及当运行SQL语句时哪种策略预计会被优化器采用，在优化慢查询时非常有用

执行explain之后结果集包含如下信息：

1
2
3

+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------+

下面将对每一个值进行解释

1、id

id用来标识整个查询中SELELCT语句的顺序，在嵌套查询中id越大的语句越先执行，该值可能为NULL; id如果相同，从上往下依次执行。id不同，id值越大，执行优先级越高，如果行引用其他行的并集结果，则该值可以为NULL。

2、select_type

select_type表示查询使用的类型，有下面几种：

2.1、simple

简单的select查询，没有union或者子查询

mysql> explain select * from  test where id = 1000;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | test  | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+

2.2、primary

最外层的select查询

mysql> explain select * from (select * from test where id = 1000) a;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
|  1 | PRIMARY     | <derived2> | system | NULL          | NULL    | NULL    | NULL  |    1 | NULL  |
|  2 | DERIVED     | test       | const  | PRIMARY       | PRIMARY | 8       | const |    1 | NULL  |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+

2.3、union

union中的第二个或随后的select查询，不依赖于外部查询的结果集

mysql> explain select * from test where id = 1000 union all select * from test2 ;
+----+--------------+------------+-------+---------------+---------+---------+-------+-------+-----------------+
| id | select_type  | table      | type  | possible_keys | key     | key_len | ref   | rows  | Extra           |
+----+--------------+------------+-------+---------------+---------+---------+-------+-------+-----------------+
|  1 | PRIMARY      | test       | const | PRIMARY       | PRIMARY | 8       | const |     1 | NULL            |
|  2 | UNION        | test2      | ALL   | NULL          | NULL    | NULL    | NULL  | 67993 | NULL            |
| NULL | UNION RESULT | <union1,2> | ALL   | NULL          | NULL    | NULL    | NULL  |  NULL | Using temporary |
+----+--------------+------------+-------+---------------+---------+---------+-------+-------+-----------------+

2.4、dependent union

union中的第二个或随后的select查询，依赖于外部查询的结果集

mysql> explain select * from test where id in (select id  from test where id = 1000 union all select id from test2) ;
+----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
| id | select_type        | table      | type   | possible_keys | key     | key_len | ref   | rows  | Extra           |
+----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
|  1 | PRIMARY            | test       | ALL    | NULL          | NULL    | NULL    | NULL  | 68505 | Using where     |
|  2 | DEPENDENT SUBQUERY | test       | const  | PRIMARY       | PRIMARY | 8       | const |     1 | Using index     |
|  3 | DEPENDENT UNION    | test2      | eq_ref | PRIMARY       | PRIMARY | 8       | func  |     1 | Using index     |
| NULL | UNION RESULT       | <union2,3> | ALL    | NULL          | NULL    | NULL    | NULL  |  NULL | Using temporary |
+----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+

2.5、subquery

子查询中的第一个 select 查询，不依赖与外部查询的结果集

mysql> explain select * from test where id = (select id from test where id = 1000);
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
|  1 | PRIMARY     | test  | const | PRIMARY       | PRIMARY | 8       | const |    1 | NULL        |
|  2 | SUBQUERY    | test  | const | PRIMARY       | PRIMARY | 8       | const |    1 | Using index |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+

2.6、dependent subquery

子查询中的第一个select查询，依赖于外部查询的结果集

mysql> explain select * from test where id in (select id  from test where id = 1000 union all select id from test2) ;
+----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
| id | select_type        | table      | type   | possible_keys | key     | key_len | ref   | rows  | Extra           |
+----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
|  1 | PRIMARY            | test       | ALL    | NULL          | NULL    | NULL    | NULL  | 68505 | Using where     |
|  2 | DEPENDENT SUBQUERY | test       | const  | PRIMARY       | PRIMARY | 8       | const |     1 | Using index     |
|  3 | DEPENDENT UNION    | test2      | eq_ref | PRIMARY       | PRIMARY | 8       | func  |     1 | Using index     |
| NULL | UNION RESULT       | <union2,3> | ALL    | NULL          | NULL    | NULL    | NULL  |  NULL | Using temporary |
+----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+

2.7、derived

用于from子句中有子查询的情况，mysql会递归执行这些子查询，此结果集放在临时表中

mysql> explain select * from (select * from test2 where id = 1000)a;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
|  1 | PRIMARY     | <derived2> | system | NULL          | NULL    | NULL    | NULL  |    1 | NULL  |
|  2 | DERIVED     | test2      | const  | PRIMARY       | PRIMARY | 8       | const |    1 | NULL  |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+

3、table

table用来表示输出行所引用的表名。

4、type（重要）

type表示访问类型，下面一次解释各种类型，类型顺序从最好到最差排列。

4.1、system

表仅有一行，是 cons 类型的一个特例。因为子查询只有一行数据，模拟了单表只有一行数据，此时 type 为 system。

mysql> explain select * from (select * from test2 where id = 1000)a;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
|  1 | PRIMARY     | <derived2> | system | NULL          | NULL    | NULL    | NULL  |    1 | NULL  |
|  2 | DERIVED     | test2      | const  | PRIMARY       | PRIMARY | 8       | const |    1 | NULL  |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+

4.2、const

确定只有一行匹配的时候，mysql 优化器会在查询前读取它并且只读一次，速度非常快。

4.3、eq_ref

对于每个来自于前表的行组合，从该表中读取一行，常用在一个索引是 unique key 或者 primary key 。

4.4、ref

对于来自前面的表的行组合，所有有匹配索引值的行都从这张表中读取，如果联接只使用键的最左边的前缀，或如果键不是 unique key 或 primary key（换句话说，如果联接不能基于关键字选择单个行的话），则使用 ref。

ref 可以用于使用 = 或 <=> 操作符的带索引的列。

mysql> explain select * from test ,test2 where test.bnet_id=test2.aid;
+----+-------------+-------+------+---------------+---------+---------+-------------------+-------+-----------------------+
| id | select_type | table | type | possible_keys | key     | key_len | ref               | rows  | Extra                 |
+----+-------------+-------+------+---------------+---------+---------+-------------------+-------+-----------------------+
|  1 | SIMPLE      | test  | ALL  | NULL          | NULL    | NULL    | NULL              | 68505 | Using where           |
|  1 | SIMPLE      | test2 | ref  | idx_aid       | idx_aid | 5       | test.test.bnet_id | 34266 | Using index condition |
+----+-------------+-------+------+---------------+---------+---------+-------------------+-------+-----------------------+

test表bnet_id不是索引，test2表aid为索引列。

4.5、ref_or_null

类似 ref, 但是添加了可以专门搜索 null 值的行。

mysql> explain select * from test where bnet_id=1 or bnet_id is null;
+----+-------------+-------+-------------+---------------+----------+---------+-------+------+-----------------------+
| id | select_type | table | type        | possible_keys | key      | key_len | ref   | rows | Extra                 |
+----+-------------+-------+-------------+---------------+----------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | test  | ref_or_null | idx_bnet      | idx_bnet | 9       | const |    2 | Using index condition |
+----+-------------+-------+-------------+---------------+----------+---------+-------+------+-----------------------+

前提为bnet_id列为索引，且bnet_id列有null值

4.6、index_merge

该访问类型使用了索引合并优化方法，key 列包含了使用的索引的清单，key_len 包含了使用的索引的最长的关键元素。

mysql> explain select * from test where id  = 1 or bnet_id = 1;
+----+-------------+-------+-------------+------------------+------------------+---------+------+------+--------------------------------------------+
| id | select_type | table | type        | possible_keys    | key              | key_len | ref  | rows | Extra                                      |
+----+-------------+-------+-------------+------------------+------------------+---------+------+------+--------------------------------------------+
|  1 | SIMPLE      | test  | index_merge | PRIMARY,idx_bnet | PRIMARY,idx_bnet | 8,9     | NULL |    2 | Using union(PRIMARY,idx_bnet); Using where |
+----+-------------+-------+-------------+------------------+------------------+---------+------+------+--------------------------------------------+

前提条件为id列和bnet_id列都有单列索引。如果出现index_merge，并且这类SQL后期使用较频繁，可以考虑把单列索引换为组合索引，这样效率更高

4.7、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。key_len 包含所使用索引的最长关键字元素。该类型中ref 列为 NULL。

当使用=、<>、>、>=、<、<=、IS NULL、<=>、BETWEEN或者IN操作符，用常量比较关键字列时，可以使用range。

mysql> explain select * from test where bnet_id > 1000 and  bnet_id < 10000;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-----------------------+
| id | select_type | table | type  | possible_keys | key      | key_len | ref  | rows | Extra                 |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-----------------------+
|  1 | SIMPLE      | test  | range | idx_bnet      | idx_bnet | 9       | NULL |    1 | Using index condition |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-----------------------+

前提条件为bnet_id列有索引。

4.8、index

在进行统计时非常常见，此联接类型实际上会扫描索引树

mysql> explain select count(*) from test;
+----+-------------+-------+-------+---------------+----------+---------+------+-------+-------------+
| id | select_type | table | type  | possible_keys | key      | key_len | ref  | rows  | Extra       |
+----+-------------+-------+-------+---------------+----------+---------+------+-------+-------------+
|  1 | SIMPLE      | test  | index | NULL          | idx_bnet | 9       | NULL | 68505 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+-------+-------------+

4.9、all

对于每个来自于先前的表的行组合，进行完整的表扫描，通常可以增加更多的索引而不要使用ALL，使得行能基于前面的表中的常数值或列值被检索出

mysql> explain select *  from test where create_time = '0000-00-00 00:00:00';
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
|  1 | SIMPLE      | test  | ALL  | NULL          | NULL | NULL    | NULL | 68505 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+

5、possible_keys

possible_keys 是指在这个 SQL 中，mysql 可以使用这个索引去辅助查找记录，当查询涉及到的字段，都会被列出，但不一定被查询使用.若为空则表示没有可以使用的索引，此时可以通过检查 where 语句看是否可以引用某些列或者新建索引来提高性能。

6、key(重要)

key 列显示的是当前表实际使用的索引，如果没有选择索引，则此列为 null，要想强制 MySQL 使用或忽视 possible_keys 列中的索引，在查询中使用 FORCE INDEX、USE INDEX 或者 IGNORE INDEX。

7、key_len

key_len 列显示 MySQL 决定使用的键长度。如果 KEY 键是 NULL，则长度为 NULL。在不损失精确性的情况下，长度越短越好key len 的长度还和字符集有关, latin1一个字符占用1个字节, gbk 一个字符占用2个字节, utf8 一个字符占用3个字节。key_len 的计算法方法如下：

列类型	长度	备注
id int	4+1	int为4bytes,允许为NULL,加1byte
id bigint not null	8	bigint为8bytes
user char(30) utf8	30*3+1	utf8每个字符为3bytes,允许为NULL,加1byte
user varchar(30) not null utf8	30*3+2	utf8每个字符为3bytes,变长数据类型,加2bytes
user varchar(30) utf8	30*3+2+1	utf8每个字符为3bytes,允许为NULL,加1byte,变长数据类型,加2bytes
detail text(10) utf8	30*3+2+1	TEXT截取部分,被视为动态列类型。

key_len只指示了where中用于条件过滤时被选中的索引列，是不包含order by或group by这一部分被选中的索引列。

8、ref

ref列用来显示使用哪个列或常数与key一起从表中选择相应的行。它显示的列的名字（或const），此列多数时候为null。

9、rows

rows列显示的是mysql解析器认为执行此SQL时必须扫描的行数。此数值为一个预估值，不是具体值，通常比实际值小。

10、filtered

此参数为mysql 5.7 新加参数，指的是返回结果的行数所占需要读到的行（rows的值）的比例，对于使用join时，前一个表的结果集大小直接影响了循环的行数。

11、extra(重要)

extra表示不在其他列并且也很重要的额外信息。

11.1、using index

该值表示这个SQL语句使用了覆盖索引（覆盖索引是指可以直接在索引列中得到想要的结果，而不用去回表），此时效率最高。

mysql> explain select id from test;
+----+-------------+-------+-------+---------------+----------+---------+------+-------+-------------+
| id | select_type | table | type  | possible_keys | key      | key_len | ref  | rows  | Extra       |
+----+-------------+-------+-------+---------------+----------+---------+------+-------+-------------+
|  1 | SIMPLE      | test  | index | NULL          | idx_bnet | 9       | NULL | 68505 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+-------+-------------+

这个例子中id字段为主键，但是key那里显示走的并不是主键索引，这个是因为mysql的所有二级索引中都会包含所有的主键信息，而mysql没有单独的存储主键索引，所以扫描二级索引的开销比全表扫描更快

11.2、using where

表示存储引擎搜到记录后进行了后过滤(POST-FILTER)，如果查询未能使用索引，using where的作用只是提醒我们mysql要用where条件过滤结果集。

mysql> explain select * from test where id > 1;
+----+-------------+-------+-------+---------------+---------+---------+------+-------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows  | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+-------+-------------+
|  1 | SIMPLE      | test  | range | PRIMARY       | PRIMARY | 8       | NULL | 34252 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+-------+-------------+

11.3、using temporary

表示mysql需要使用临时表来存储结果集，常见于排序和分组查询。

mysql> explain select * from test where id in (1,2) group by bnet_id;
+----+-------------+-------+-------+-----------------------------------------+---------+---------+------+------+----------------------------------------------+
| id | select_type | table | type  | possible_keys                           | key     | key_len | ref  | rows | Extra                                        |
+----+-------------+-------+-------+-----------------------------------------+---------+---------+------+------+----------------------------------------------+
|  1 | SIMPLE      | test  | range | PRIMARY,IDX(event_key-bnet_Id),idx_bnet | PRIMARY | 8       | NULL |    2 | Using where; Using temporary; Using filesort |
+----+-------------+-------+-------+-----------------------------------------+---------+---------+------+------+----------------------------------------------+

11.4、using filesort

是指mysql无法利用索引直接完成排序（排序的字段不是索引字段），此时会用到缓冲空间来进行排序。

mysql> explain select * from test order by bnet_id;
+----+-------------+-------+------+---------------+------+---------+------+-------+----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra          |
+----+-------------+-------+------+---------------+------+---------+------+-------+----------------+
|  1 | SIMPLE      | test  | ALL  | NULL          | NULL | NULL    | NULL | 68505 | Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+-------+----------------+

11.5、using join buffer

强调在获取连接条件时没有用到索引，并且需要连接缓冲区来存储中间结果。（性能可以通过添加索引或者修改连接字段改进）。

mysql> explain select * from test left join test2 on test.create_time = test2.create_time;
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra                                              |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------------------------------------------+
|  1 | SIMPLE      | test  | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 959692 |   100.00 | NULL                                               |
|  1 | SIMPLE      | test2 | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 958353 |   100.00 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------------------------------------------+
2 rows in set, 1 warning (0.00 sec)

Block Nested Loop 是指 Block Nested-Loop Join算法：将外层循环的行/结果集存入 join buffer, 内层循环的每一行与整个buffer中的记录做比较，从而减少内层循环的次数。

11.6、impossible where

表示where条件导致没有返回的行。

mysql> explain select * from test where id is null;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra            |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+