MySQL Explain 执行计划

发表于 2020-06-16 分类于 MySql

Explain有什么用

当Explain与SQL语句一起使用时，MySQL会显示来自优化器关于SQL执行的信息。
也就是说，MySQL解释了它将如何处理该语句，包括如何连接表以及什么顺序连接表等。

表的加载顺序
sql的查询类型
可能用到哪些索引，哪些索引又被实际使用
表与表之间的引用关系
一个表中有多少行被优化器查询

Explain有哪些信息

Explain执行计划包含字段信息如下：
分别是 id、select_type、table、partitions、type、possible_keys、key、key_len、ref、rows、filtered、Extra12个字段。

下边会结合具体的SQL示例，详细的解读每个字段以及每个字段中不同参数的含义，以下所有示例数据库版本为 MySQL.5.7.18

mysql> select version();
+-----------+
| version() |
+-----------+
| 5.7.18    |
+-----------+

创建三张表 one、two、three，
表之间的关系 one.two_id = two.two_id AND two.three_id = three.three_id

Explain执行计划详解

id

id: 表示查询中执行select子句或者操作表的顺序，**id的值越大，代表优先级越高，越先执行**。id大致会出现 3种情况：

id相同
看到三条记录的id都相同，可以理解成这三个表为一组，具有同样的优先级，执行顺序由上而下，具体顺序由优化器决定。

mysql> EXPLAIN SELECT * FROM one o,two t, three r WHERE o.two_id = t.two_id AND t.three_id = r.three_id;
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref                  | rows | filtered | Extra                                              |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+
|  1 | SIMPLE      | o     | NULL       | ALL    | NULL          | NULL    | NULL    | NULL                 |    2 |      100 | NULL                                               |
|  1 | SIMPLE      | t     | NULL       | ALL    | PRIMARY       | NULL    | NULL    | NULL                 |    2 |       50 | Using where; Using join buffer (Block Nested Loop) |
|  1 | SIMPLE      | r     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | xin-slave.t.three_id |    1 |      100 | NULL                                               |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+

id不同
如果我们的SQL中存在子查询，那么id的序号会递增，id值越大优先级越高，越先被执行。
当三个表依次嵌套，发现最里层的子查询id最大，最先执行。

mysql> EXPLAIN select * from one o where o.two_id = (select t.two_id from two t where t.three_id = (select r.three_id  from three r where r.three_name='我是第三表2'));
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | PRIMARY     | o     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |       50 | Using where |
|  2 | SUBQUERY    | t     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |       50 | Using where |
|  3 | SUBQUERY    | r     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |       50 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

以上两种同时存在
将上边的SQL稍微修改一下，增加一个子查询，发现id的以上两种同时存在。
相同id划分为一组，这样就有三个组，同组的从上往下顺序执行，不同组id值越大，优先级越高，越先执行。

mysql>  EXPLAIN select * from one o where o.two_id = (select t.two_id from two t where t.three_id = (select r.three_id  from three r where r.three_name='我是第三表2')) AND o.one_id in(select one_id from one where o.one_name="我是第一表2");
+----+-------------+-------+------------+--------+---------------+---------+---------+--------------------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref                | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+---------------+---------+---------+--------------------+------+----------+-------------+
|  1 | PRIMARY     | o     | NULL       | ALL    | PRIMARY       | NULL    | NULL    | NULL               |    2 |       50 | Using where |
|  1 | PRIMARY     | one   | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | xin-slave.o.one_id |    1 |      100 | Using index |
|  2 | SUBQUERY    | t     | NULL       | ALL    | NULL          | NULL    | NULL    | NULL               |    2 |       50 | Using where |
|  3 | SUBQUERY    | r     | NULL       | ALL    | NULL          | NULL    | NULL    | NULL               |    2 |       50 | Using where |
+----+-------------+-------+------------+--------+---------------+---------+---------+--------------------+------+----------+-------------+

select_type

select_type：表示select查询的类型，主要是用于区分各种复杂的查询，例如：普通查询、联合查询、子查询等。

SIMPLE
SIMPLE：表示最简单的select查询语句，也就是在查询中不包含子查询或者union交并差集等操作。
PRIMARY
PRIMARY：当查询语句中包含任何复杂的子部分，最外层查询则被标记为PRIMARY。
SUBQUERY
SUBQUERY：当select或where列表中包含了子查询，该子查询被标记为：SUBQUERY。
DERIVED
DERIVED：表示包含在from子句中的子查询的select，在我们的from列表中包含的子查询会被标记为derived。
UNION
UNION：如果union后边又出现的select语句，则会被标记为union；
若union包含在from子句的子查询中，外层select将被标记为derived。

UNION RESULT
UNION RESULT：代表从union的临时表中读取数据，
而table列的<union1,4>表示用第一个和第四个select的结果进行union操作。

mysql> EXPLAIN select t.two_name, ( select one.one_id from one) o from (select two_id,two_name from two where two_name ='') t  union (select r.three_name,r.three_id from three r);

+------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
| id   | select_type  | table      | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra           |
+------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
|    1 | PRIMARY      | two        | NULL       | ALL   | NULL          | NULL    | NULL    | NULL |    2 |       50 | Using where     |
|    2 | SUBQUERY     | one        | NULL       | index | NULL          | PRIMARY | 4       | NULL |    2 |      100 | Using index     |
|    4 | UNION        | r          | NULL       | ALL   | NULL          | NULL    | NULL    | NULL |    2 |      100 | NULL            |
| NULL | UNION RESULT | <union1,4> | NULL       | ALL   | NULL          | NULL    | NULL    | NULL | NULL | NULL     | Using temporary |
+------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+

table

查询的表名，并不一定是真实存在的表，有别名显示别名，也可能为临时表，例如上边的DERIVED、<union1,4>等。

partitions

查询时匹配到的分区信息，对于非分区表值为NULL，当查询的是分区表时，partitions显示分区表命中的分区情况。

+----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table          | partitions                      | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | one            | p201801,p201802,p201803,p300012 | index | NULL          | PRIMARY | 9       | NULL |    3 |      100 | Using index |
+----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+

type

type：查询使用了何种类型，它在SQL优化中是一个非常重要的指标，
以下性能从好到坏依次是：
system > const > eq_ref > ref > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL

system
system：当表仅有一行记录时(系统表)，数据量很少，往往不需要进行磁盘IO，速度非常快。

const
const：表示查询时命中primary key主键或者unique唯一索引，或者被连接的部分是一个常量(const)值。
这类扫描效率极高，返回数据量少，速度非常快。

mysql> EXPLAIN SELECT * from three where three_id=1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | three | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |      100 | NULL  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+

eq_ref
eq_ref：查询时命中主键primary key或者unique key索引，type就是eq_ref。

mysql> EXPLAIN select o.one_name from one o ,two t where o.one_id = t.two_id ; 
+----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys | key      | key_len | ref                | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+
|  1 | SIMPLE      | o     | NULL       | index  | PRIMARY       | idx_name | 768     | NULL               |    2 |      100 | Using index |
|  1 | SIMPLE      | t     | NULL       | eq_ref | PRIMARY       | PRIMARY  | 4       | xin-slave.o.one_id |    1 |      100 | Using index |
+----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+

ref
ref：区别于eq_ref，ref表示使用非唯一性索引，会找到很多个符合条件的行。

mysql> select o.one_id from one o where o.one_name = "xin" ; 
+--------+
| one_id |
+--------+
|      1 |
|      3 |
+--------+

mysql> EXPLAIN select o.one_id from one o where o.one_name = "xin" ; 
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key      | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | o     | NULL       | ref  | idx_name      | idx_name | 768     | const |    1 |      100 | Using index |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+

ref_or_null
ref_or_null：这种连接类型类似于 ref，区别在于MySQL会额外搜索包含NULL值的行。

mysql> EXPLAIN select o.one_id from one o where o.one_name = "xin" OR o.one_name IS NULL; 
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+
| id | select_type | table | partitions | type        | possible_keys | key      | key_len | ref   | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+
|  1 | SIMPLE      | o     | NULL       | ref_or_null | idx_name      | idx_name | 768     | const |    3 |      100 | Using where; Using index |
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+

index_merge
index_merge：使用了索引合并优化方法，查询使用了两个以上的索引。
下边示例中同时使用到主键one_id和字段one_name的idx_name索引。

mysql> EXPLAIN select * from one o where o.one_id >1 and o.one_name ='xin'; 
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+
| id | select_type | table | partitions | type        | possible_keys    | key              | key_len | ref  | rows | filtered | Extra                                          |
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+
|  1 | SIMPLE      | o     | NULL       | index_merge | PRIMARY,idx_name | idx_name,PRIMARY | 772,4   | NULL |    1 |      100 | Using intersect(idx_name,PRIMARY); Using where |
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+

unique_subquery
unique_subquery：替换下面的IN子查询，子查询返回不重复的集合。
value IN (SELECT primary_key FROM single_table WHERE some_expr)
index_subquery
index_subquery：区别于unique_subquery，用于非唯一索引，可以返回重复值。
value IN (SELECT key_column FROM single_table WHERE some_expr)

range
range：使用索引选择行，仅检索给定范围内的行。简单点说就是针对一个有索引的字段，给定范围检索数据。
在where语句中使用bettween...and、<、>、<=、in等条件查询type都是range。
举个栗子：three表中three_id为唯一主键，user_id普通字段未建索引。

mysql> EXPLAIN SELECT * from three where three_id BETWEEN 2 AND 3;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | three | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |    1 |      100 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

从结果中看到只有对设置了索引的字段，做范围检索type才是range。

mysql> EXPLAIN SELECT * from three where user_id BETWEEN 2 AND 3;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | three | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

index
index：Index与ALL其实都是读全表，区别在于index是遍历索引树读取，而ALL是从硬盘中读取。
下边示例：three_id为主键，不带where条件全表查询，type结果为index。

mysql> EXPLAIN SELECT three_id from three ;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | three | NULL       | index | NULL          | PRIMARY | 4       | NULL |    1 |      100 | Using index |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

ALL
ALL：将遍历全表以找到匹配的行，性能最差。

mysql> EXPLAIN SELECT * from two ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | two   | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |      100 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+

possible_keys

possible_keys：表示在MySQL中通过哪些索引，能让我们在表中找到想要的记录，一旦查询涉及到的某个字段上存在索引，
则索引将被列出，但这个索引并不定一会是最终查询数据时所被用到的索引。具体请参考上边的例子。

key

key：区别于possible_keys，key是查询中实际使用到的索引，若没有使用索引，显示为NULL。具体请参考上边的例子。

当type为index_merge时，可能会显示多个索引。

key_len

key_len：表示查询用到的索引长度（字节数），原则上长度越短越好。

单列索引，那么需要将整个索引长度算进去；
多列索引，不是所有列都能用到，需要计算查询中实际用到的列。
注意：key_len只计算where条件中用到的索引长度，而排序和分组即便是用到了索引，也不会计算到key_len中。

ref

ref：常见的有：const，func，null，字段名。

当使用常量等值查询，显示const，
当关联查询时，会显示相应关联表的关联字段
如果查询条件使用了表达式、函数，或者条件列发生内部隐式转换，可能显示为func
其他情况null

rows

rows：以表的统计信息和索引使用情况，估算要找到我们所需的记录，需要读取的行数。
这是评估SQL性能的一个比较重要的数据，mysql需要扫描的行数，很直观的显示SQL性能的好坏，一般情况下rows值越小越好。

mysql> EXPLAIN SELECT * from three;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | three | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |      100 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+

filtered

filtered这个是一个百分比的值，表里符合条件的记录数的百分比。
简单点说，这个字段表示存储引擎返回的数据在经过过滤后，剩下满足条件的记录数量的比例。

在MySQL.5.7版本以前想要显示filtered需要使用explain extended命令。
MySQL.5.7后，默认explain直接显示partitions和filtered的信息。

Extra

Extra：不适合在其他列中显示的信息，Explain中的很多额外的信息会在Extra字段显示。

Using index
Using index：我们在相应的select操作中使用了覆盖索引，通俗一点讲就是查询的列被索引覆盖，
使用到覆盖索引查询速度会非常快，SQl优化中理想的状态。
什么又是覆盖索引?
一条SQL只需要通过索引就可以返回，我们所需要查询的数据（一个或几个字段），而不必通过二级索引，
查到主键之后再通过主键查询整行数据（select *）。

-- one_id表为主键
mysql> EXPLAIN SELECT one_id from one ;
+----+-------------+-------+------------+-------+---------------+------------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key        | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+------------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | one   | NULL       | index | NULL          | idx_two_id | 5       | NULL |    3 |      100 | Using index |
+----+-------------+-------+------------+-------+---------------+------------+---------+------+------+----------+-------------+
```        
注意：想要使用到覆盖索引，我们在`select`时只取出需要的字段，不可`select *`，而且该字段建了索引。    
```mysql
mysql> EXPLAIN SELECT * from one ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | one   | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |      100 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+

Using where
Using where：查询时未找到可用的索引，进而通过where条件过滤获取所需数据，
但要注意的是并不是所有带where语句的查询都会显示Using where。
下边示例create_time并未用到索引，type为ALL，即MySQL通过全表扫描后再按where条件筛选数据。

mysql> EXPLAIN SELECT one_name from one where create_time ='2020-05-18';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | one   | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

Using temporary
Using temporary：表示查询后结果需要使用临时表来存储，一般在排序或者分组查询时用到。

mysql> EXPLAIN SELECT one_name from one where one_id in (1,2) group by one_name;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | one   | NULL       | range| NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where; Using temporary; Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

Using filesort
Using filesort：表示无法利用索引完成的排序操作，也就是ORDER BY的字段没有索引，通常这样的SQL都是需要优化的。

mysql> EXPLAIN SELECT one_id from one  ORDER BY create_time;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
|  1 | SIMPLE      | one   | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |      100 | Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+

如果ORDER BY字段有索引就会用到覆盖索引，相比执行速度快很多。

mysql> EXPLAIN SELECT one_id from one  ORDER BY one_id;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | one   | NULL       | index | NULL          | PRIMARY | 4       | NULL |    3 |      100 | Using index |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

Using join buffer
Using join buffer：在我们联表查询的时候，如果表的连接条件没有用到索引，需要有一个连接缓冲区来存储中间结果。
先看一下有索引的情况：连接条件one_name、two_name都用到索引。

mysql> EXPLAIN SELECT one_name from one o,two t where o.one_name = t.two_name;
+----+-------------+-------+------------+-------+---------------+----------+---------+----------------------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key      | key_len | ref                  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+----------+---------+----------------------+------+----------+--------------------------+
|  1 | SIMPLE      | o     | NULL       | index | idx_name      | idx_name | 768     | NULL                 |    3 |      100 | Using where; Using index |
|  1 | SIMPLE      | t     | NULL       | ref   | idx_name      | idx_name | 768     | xin-slave.o.one_name |    1 |      100 | Using index              |
+----+-------------+-------+------------+-------+---------------+----------+---------+----------------------+------+----------+--------------------------+

接下来删掉连接条件one_name、two_name的字段索引。
发现Extra列变成Using join buffer，type均为全表扫描，这也是SQL优化中需要注意的地方。

mysql> EXPLAIN SELECT one_name from one o,two t where o.one_name = t.two_name;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                                              |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
|  1 | SIMPLE      | t     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    2 |      100 | NULL                                               |
|  1 | SIMPLE      | o     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+

Impossible where
Impossible where：表示在我们用不太正确的where语句，导致没有符合条件的行。

mysql> EXPLAIN SELECT one_name from one WHERE 1=2;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
| 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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+

No tables used
No tables used：我们的查询语句中没有FROM子句，或者有FROM DUAL子句。

mysql> EXPLAIN select now();
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| 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     | No tables used |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+

Extra 列的信息非常非常多，这里就不再一一列举了，详见MySQL官方文档：https://dev.mysql.com/doc/ref...

总结

重点看的几列：

possible_keys：可能可以利用的索引的名字。这里的索引名字是创建索引时指定的索引昵称；
如果索引没有昵称，则默认显示的是索引中第一个列的名字（在本例中，它是“firstname”）。
默认索引名字的含义往往不是很明显。
key：它显示了MySQL实际使用的索引的名字。如果它为空（或NULL），则MySQL不使用索引。
key_len：索引中被使用部分的长度，以字节计。
ref：列出是通过常量（const），还是某个表的某个字段（如果是join）来过滤（通过key）的。
rows：MySQL所认为的它在找到正确的结果之前必须扫描的记录数。显然，这里最理想的数字就是1。