原文地址:https://www.cnblogs.com/gomysql/p/3720123.html

在工作中,我们用于捕捉性能问题最常用的就是打开慢查询,定位执行效率差的SQL,那么当我们定位到一个SQL以后还不算完事,我们还需要知道该SQL的执行计划,比如是全表扫描,还是索引扫描,这些都需要通过EXPLAIN去完成。EXPLAIN命令是查看优化器如何决定执行查询的主要方法。可以帮助我们深入了解MySQL的基于开销的优化器,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。需要注意的是,生成的QEP并不确定,它可能会根据很多因素发生改变。MySQL不会将一个QEP和某个给定查询绑定,QEP将由SQL语句每次执行时的实际情况确定,即便使用存储过程也是如此。尽管在存储过程中SQL语句都是预先解析过的,但QEP仍然会在每次调用存储过程的时候才被确定。

通过执行计划可以知道什么?

mysql> explain select d1.age, t2.id from (select age,name from t1 where id in (1,2))d1, t2 where d1.age=t2.age group by d1.age, t2.id order by t2.id;
+----+-------------+------------+-------+---------------+---------+---------+--------+------+---------------------------------+
| id | select_type | table      | type  | possible_keys | key     | key_len | ref    | rows | Extra                           |
+----+-------------+------------+-------+---------------+---------+---------+--------+------+---------------------------------+
|  1 | PRIMARY     | <derived2> | ALL   | NULL          | NULL    | NULL    | NULL   |    2 | Using temporary; Using filesort |
|  1 | PRIMARY     | t2         | ref   | age           | age     | 5       | d1.age |    1 | Using where; Using index        |
|  2 | DERIVED     | t1         | range | PRIMARY       | PRIMARY | 4       | NULL   |    2 | Using where                     |
+----+-------------+------------+-------+---------------+---------+---------+--------+------+---------------------------------+
3 rows in set (0.00 sec)

mysql> 

MySQL执行计划调用方式
1.EXPLAIN SELECT ……
变体:
2.EXPLAIN EXTENDED SELECT ……
将执行计划"反编译"成SELECT语句,运行SHOW WARNINGS
可得到被MySQL优化器优化后的查询语句
3.EXPLAIN PARTITIONS SELECT ……
用于分区表的EXPLAIN生成QEP的信息

执行计划包含的信息

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

1. id:

包含一组数字,表示查询中执行select子句或操作表的顺序

Example(id相同,执行顺序由上至下

mysql> explain select t2.* from t1, t2, t3 where t1.id=t2.id and t1.id=t3.id and t1.name='';
+----+-------------+-------+--------+---------------+---------+---------+------------+------+--------------------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref        | rows | Extra                    |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+--------------------------+
|  1 | SIMPLE      | t1    | ref    | PRIMARY,name  | name    | 63      | const      |    1 | Using where; Using index |
|  1 | SIMPLE      | t2    | eq_ref | PRIMARY       | PRIMARY | 4       | test.t1.id |    1 |                          |
|  1 | SIMPLE      | t3    | eq_ref | PRIMARY       | PRIMARY | 4       | test.t1.id |    1 | Using index              |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+--------------------------+
3 rows in set (0.00 sec)

mysql> 

Example (如果是子查询,id的序号会递增,id值越大优先级越高,越先被执行)

mysql> explain select t2.* from t2 where id = (select id from t1 where id = (select t3.id from t3 where t3.name=''));
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                               |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
|  1 | PRIMARY     | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE noticed after reading const tables |
|  2 | SUBQUERY    | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | no matching row in const table                      |
|  3 | SUBQUERY    | t3    | ref  | name          | name | 63      |      |    1 | Using where; Using index                            |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
3 rows in set (0.00 sec)

mysql> 

Example(id如果相同,可以认为是一组,从上往下顺序执行;在所有组中,id值越大,优先级越高,越先执行)

mysql> explain select t2.* from (select t3.id from t3 where t3.name='')s1, t2 where s1.id=t2.id;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+--------------------------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref   | rows | Extra                    |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+--------------------------+
|  1 | PRIMARY     | <derived2> | system | NULL          | NULL    | NULL    | NULL  |    1 |                          |
|  1 | PRIMARY     | t2         | const  | PRIMARY       | PRIMARY | 4       | const |    1 |                          |
|  2 | DERIVED     | t3         | ref    | name          | name    | 63      |       |    1 | Using where; Using index |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+--------------------------+
3 rows in set (0.00 sec)

mysql> 

2. select_type

示查询中每个select子句的类型(简单OR复杂)

a. SIMPLE:查询中不包含子查询或者UNION
b. 查询中若包含任何复杂的子部分,最外层查询则被标记为:PRIMARY
c. 在SELECT或WHERE列表中包含了子查询,该子查询被标记为:SUBQUERY
d. 在FROM列表中包含的子查询被标记为:DERIVED(衍生)用来表示包含在from子句中的子查询的select,mysql会递归执行并将结果放到一个临时表中。服务器内部称为"派生表",因为该临时表是从子查询中派生出来的
e. 若第二个SELECT出现在UNION之后,则被标记为UNION;若UNION包含在FROM子句的子查询中,外层SELECT将被标记为:DERIVED
f. 从UNION表获取结果的SELECT被标记为:UNION RESULT

SUBQUERY和UNION还可以被标记为DEPENDENT和UNCACHEABLE。
DEPENDENT意味着select依赖于外层查询中发现的数据。
UNCACHEABLE意味着select中的某些 特性阻止结果被缓存于一个item_cache中。

Example

mysql> explain select d1.name, ( select id from t3) d2 from (select id,name from t1 where name='')d1 union (select name,id from t2);
+------+--------------+------------+--------+---------------+------+---------+------+------+--------------------------+
| id   | select_type  | table      | type   | possible_keys | key  | key_len | ref  | rows | Extra                    |
+------+--------------+------------+--------+---------------+------+---------+------+------+--------------------------+
|  1   | PRIMARY      | <derived3> | system | NULL          | NULL | NULL    | NULL |    0 | const row not found      |
|  3   | DERIVED      | t1         | ref    | name          | name | 63      |      |    1 | Using where; Using index |
|  2   | SUBQUERY     | t3         | index  | NULL          | age  | 5       | NULL |    6 | Using index              |
|  4   | UNION        | t2         | index  | NULL          | name | 63      | NULL |    4 | Using index              |
| NULL | UNION RESULT | <union1,4> | ALL    | NULL          | NULL | NULL    | NULL | NULL |                          |
+------+--------------+------------+--------+---------------+------+---------+------+------+--------------------------+
5 rows in set (0.00 sec)

mysql> 

第一行:id列为1,表示第一个select,select_type列的primary表 示该查询为外层查询,table列被标记为<derived3>,表示查询结果来自一个衍生表,其中3代表该查询衍生自第三个select查询,即id为3的select。
第二行:id为3,表示该查询的执行次序为2( 4 => 3),是整个查询中第三个select的一部分。因查询包含在from中,所以为derived。
第三行:select列表中的子查询,select_type为subquery,为整个查询中的第二个select。
第四行:select_type为union,说明第四个select是union里的第二个select,最先执行。
第五行:代表从union的临时表中读取行的阶段,table列的<union1,4>表示用第一个和第四个select的结果进行union操作。

3. type                                       

表示MySQL在表中找到所需行的方式,又称“访问类型”,常见类型如下:

 ALL, index,  range, ref, eq_ref, const, system, NULL

从左到右,性能从最差到最好

Example

a. ALL:Full Table Scan, MySQL将遍历全表以找到匹配的行

mysql> explain select * from t1 where email='';
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    4 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> 

b. index:Full Index Scan,index与ALL区别为index类型只遍历索引树

mysql> explain select id from t1;
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | index | NULL          | age  | 5       | NULL |    4 | Using index |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> 

c. range:索引范围扫描,对索引的扫描开始于某一点,返回匹配值域的行。显而易见的索引范围扫描是带有between或者where子句里带有<, >查询。当mysql使用索引去查找一系列值时,例如IN()和OR列表,也会显示range(范围扫描),当然性能上面是有差异的。

mysql> explain select * from t1 where id in (1,4);
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from t1 where id between 1 and 4;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | range | PRIMARY       | PRIMARY | 4       | NULL |    3 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from t1 where id=1 or id=4;       
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.01 sec)

mysql> explain select * from t1 where id > 1;      
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | range | PRIMARY       | PRIMARY | 4       | NULL |    3 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> 

d. ref:使用非唯一索引扫描或者唯一索引的前缀扫描,返回匹配某个单独值的记录行

mysql> explain select * from t1 where name='yayun';
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref   | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
|  1 | SIMPLE      | t1    | ref  | name          | name | 63      | const |    1 | Using where |
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
1 row in set (0.00 sec)

mysql> 

e. eq_ref:类似ref,区别就在使用的索引是唯一索引,对于每个索引键值,表中只有一条记录匹配,简单来说,就是多表连接中使用primary key或者 unique key作为关联条件

mysql> explain select t1.name from t1, t2 where t1.id=t2.id;
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref        | rows | Extra       |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
|  1 | SIMPLE      | t1    | index  | PRIMARY       | name    | 63      | NULL       |    4 | Using index |
|  1 | SIMPLE      | t2    | eq_ref | PRIMARY       | PRIMARY | 4       | test.t1.id |    1 | Using index |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
2 rows in set (0.00 sec)

mysql> 

f. const、system:当MySQL对查询某部分进行优化,并转换为一个常量时,使用这些类型访问。如将主键置于where列表中,MySQL就能将该查询转换为一个常量

mysql> explain select * from ( select * from t1 where id=1)b1;
+----+-------------+------------+--------+---------------+---------+---------+------+------+-------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref  | rows | Extra |
+----+-------------+------------+--------+---------------+---------+---------+------+------+-------+
|  1 | PRIMARY     | <derived2> | system | NULL          | NULL    | NULL    | NULL |    1 |       |
|  2 | DERIVED     | t1         | const  | PRIMARY       | PRIMARY | 4       |      |    1 |       |
+----+-------------+------------+--------+---------------+---------+---------+------+------+-------+
2 rows in set (0.00 sec)

mysql> 

注:system是const类型的特例,当查询的表只有一行的情况下,使用system

g. NULL:MySQL在优化过程中分解语句,执行时甚至不用访问表或索引,例如从一个索引列里选取最小值可以通过单独索引查找完成。

mysql> explain select * from t1 where id = (select min(id) from t2);
+----+-------------+-------+-------+---------------+---------+---------+-------+------+------------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra                        |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+------------------------------+
|  1 | PRIMARY     | t1    | const | PRIMARY       | PRIMARY | 4       | const |    1 |                              |
|  2 | SUBQUERY    | NULL  | NULL  | NULL          | NULL    | NULL    | NULL  | NULL | Select tables optimized away |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+------------------------------+
2 rows in set (0.00 sec)

mysql> 

4. possible_keys
指出MySQL能使用哪个索引在表中找到记录,查询涉及到的字段上若存在索引,则该索引将被列出,但不一定被查询使用


5. key
显示MySQL在查询中实际使用的索引,若没有使用索引,显示为NULL

Example

mysql> explain select id,age from t1;
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | index | NULL          | age  | 5       | NULL |    4 | Using index |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> 

6. key_len
表示索引中使用的字节数,可通过该列计算查询中使用的索引的长度(key_len显示的值为索引字段的最大可能长度,并非实际使用长度,即key_len是根据表定义计算而得,不是通过表内检索出的)

7. ref
表示上述表的连接匹配条件,即哪些列或常量被用于查找索引列上的值

8. rows
表示MySQL根据表统计信息及索引选用情况,估算的找到所需的记录所需
要读取的行数

Example

mysql> explain select * from t1 , t2 where t1.id=t2.id and t2.name='atlas';
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref        | rows | Extra       |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
|  1 | SIMPLE      | t2    | ref    | PRIMARY,name  | name    | 63      | const      |    1 | Using where |
|  1 | SIMPLE      | t1    | eq_ref | PRIMARY       | PRIMARY | 4       | test.t2.id |    1 |             |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
2 rows in set (0.00 sec)

mysql> 

9. Extra
包含不适合在其他列中显示但十分重要的额外信息
a. Using index
该值表示相应的select操作中使用了覆盖索引(Covering Index)

Example

mysql> explain select id from t1;
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | index | NULL          | age  | 5       | NULL |    4 | Using index |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> 

覆盖索引(Covering Index)
MySQL可以利用索引返回select列表中的字段,而不必根据索引再次读取数据文件
包含所有满足查询需要的数据的索引称为覆盖索引(Covering Index)
注意:如果要使用覆盖索引,一定要注意select列表中只取出需要的列,不可select *,因为如果将所有字段一起做索引会导致索引文件过大,查询性能下降

b. Using where
表示mysql服务器将在存储引擎检索行后再进行过滤。许多where条件里涉及索引中的列,当(并且如果)它读取索引时,就能被存储引擎检验,因此不是所有带where字句的查询都会显示"Using where"。有时"Using where"的出现就是一个暗示:查询可受益与不同的索引。

Example

mysql> explain select id,name from t1 where id<4;
+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
|  1 | SIMPLE      | t1    | index | PRIMARY       | name | 63      | NULL |    4 | Using where; Using index |
+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

mysql> 

c. 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

这个值表示使用了内部临时(基于内存的)表。一个查询可能用到多个临时表。有很多原因都会导致MySQL在执行查询期间创建临时表。两个常见的原因是在来自不同表的上使用了DISTINCT,或者使用了不同的ORDER BY和GROUP BY列。可以强制指定一个临时表使用基于磁盘的MyISAM存储引擎。这样做的原因主要有两个:
1)内部临时表占用的空间超过min(tmp_table_size,max_heap_table_size)系统变量的限制
2)使用了TEXT/BLOB 列

Example

mysql> explain select id from t1 where id in (1,2) group by age,name;
+----+-------------+-------+-------+---------------+---------+---------+------+------+----------------------------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                                        |
+----+-------------+-------+-------+---------------+---------+---------+------+------+----------------------------------------------+
|  1 | SIMPLE      | t1    | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where; Using temporary; Using filesort |
+----+-------------+-------+-------+---------------+---------+---------+------+------+----------------------------------------------+
1 row in set (0.00 sec)

mysql> 

d. Using filesort
MySQL中无法利用索引完成的排序操作称为“文件排序”

Example

mysql> explain select id,age from t1 order by name; 
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    4 | Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)

mysql> explain select id,age from t1 order by age; 
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | index | NULL          | age  | 5       | NULL |    4 | Using index |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> 

e. Using join buffer
改值强调了在获取连接条件时没有使用索引,并且需要连接缓冲区来存储中间结果。如果出现了这个值,那应该注意,根据查询的具体情况可能需要添加索引来改进能。

Example

mysql> explain select t1.name from t1 inner join t2 on t1.name=t2.name;
+----+-------------+-------+-------+---------------+------+---------+--------------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref          | rows | Extra                    |
+----+-------------+-------+-------+---------------+------+---------+--------------+------+--------------------------+
|  1 | SIMPLE      | t1    | index | name          | name | 63      | NULL         |    4 | Using index              |
|  1 | SIMPLE      | t2    | ref   | name          | name | 63      | test.t1.name |    2 | Using where; Using index |
+----+-------------+-------+-------+---------------+------+---------+--------------+------+--------------------------+
2 rows in set (0.00 sec)

mysql> alter table t1 drop key name;                                   
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table t2 drop key name; 
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain select t1.name from t1 inner join t2 on t1.name=t2.name;
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                          |
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
|  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    4 |                                |
|  1 | SIMPLE      | t2    | ALL  | NULL          | NULL | NULL    | NULL |    4 | Using where; Using join buffer |
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
2 rows in set (0.00 sec)

mysql> 

f. Impossible where
这个值强调了where语句会导致没有符合条件的行。

Example

mysql> EXPLAIN SELECT * FROM t1 WHERE 1=2;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra            |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
1 row in set (0.00 sec)

mysql> 

h. Select tables optimized away
这个值意味着仅通过使用索引,优化器可能仅从聚合函数结果中返回一行.

Example

mysql> explain select max(id) from t1;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                        |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
1 row in set (0.00 sec)

mysql> 

I. Index merges
当MySQL 决定要在一个给定的表上使用超过一个索引的时候,就会出现以下格式中的一个,详细说明使用的索引以及合并的类型。
Using sort_union(...)
Using union(...)
Using intersect(...)

 

总结:
• EXPLAIN不会告诉你关于触发器、存储过程的信息或用户自定义函数对查询的影响情况
• EXPLAIN不考虑各种Cache
• EXPLAIN不能显示MySQL在执行查询时所作的优化工作
• 部分统计信息是估算的,并非精确值
• EXPALIN只能解释SELECT操作,其他操作要重写为SELECT后查看执行计划。