个人博客地址:http://www.chengxy-nds.top,2000G 技术资源自取
昨天中午在食堂,和部门的技术大牛们坐在一桌吃饭,作为一个卑微技术渣仔默默的吃着饭,听大佬们高谈阔论,研究各种高端技术,我TM也想说话可实在插不上嘴。
聊着聊着突然说到他上午面试了一个工作6年的程序员,表情挺复杂,他说:我看他简历写着熟悉SQL
语句调优,就问了下 Explain
执行计划怎么看?结果这老哥一问三不知,工作6年这么基础的东西都不了解!
感受到了大佬的王之鄙视,回到工位我就开始默默写这个,哎~ 我TM也不太懂 Explain
,老哥你这是针对我啊!哭唧唧~
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
、Extra
12个字段。
下边我们会结合具体的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.5.7.17
。
mysql> select version() from dual;
+------------+
| version() |
+------------+
| 5.7.17-log |
+------------+
我们创建三张表 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
一、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:
1、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 |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+
2、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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
##### 3、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 |
+----+-------------+-------+------------+--------+---------------+---------+---------+--------------------+------+----------+-------------+
二、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。
1、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。
2、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
。
3、SUBQUERY
SUBQUERY
:当 select
或 where
列表中包含了子查询,该子查询被标记为:SUBQUERY
。
4、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
。
5、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
。
6、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 |
+------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
三、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。
四、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 |
+----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+
五、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
1、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。
2、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 |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
3、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 |
+----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+
4、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 |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
5、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 |
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+
6、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 |
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+
7、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)
8、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)
9、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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
10、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 |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
11、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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
六、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。
七、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。
八、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。
九、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
十、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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
十一、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。
十二、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。
1、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> 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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
2、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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
3、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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
4、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 |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
5、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) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
6、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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
7、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
上边只是简单介绍了下 Explain
执行计划各个列的含义,了解它不仅仅是要应付面试,在实际开发中也经常会用到。比如对慢SQL
进行分析,如果连执行计划结果都不会看,那还谈什么SQL
优化呢?
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