mysql的explain

Posted true-love

tags:

篇首语:本文由小常识网(cha138.com)小编为大家整理,主要介绍了mysql的explain相关的知识,希望对你有一定的参考价值。

explain 一般用于分析sql.  
如下

[SQL] 纯文本查看 复制代码
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
drop table if exists test1;
CREATE TABLE test1 (
    id INT NOT NULL primary key  auto_increment, -- 自动递增
    username varchar(5) not null -- 用户名
)
ENGINE=innodb ;
 
insert into test1 (username) values (concat(‘test‘,rand()));
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;
insert into test1 (username)   select username from test1;


先创建一部分数据. 然后执行

[SQL] 纯文本查看 复制代码
1
explain select * from test1;


结果

explain select username from test1;
+----+-------------+-------+------+---------------+------+---------+------+---------+-------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows    | Extra |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------+
|  1 | SIMPLE      | test1 | ALL  | NULL          | NULL | NULL    | NULL | 4187248 | NULL  |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------+
1 row in set (0.00 sec)


再执行

[SQL] 纯文本查看 复制代码
1
explain select username from test1 where username = ‘test‘;

 

+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows    | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
|  1 | SIMPLE      | test1 | ALL  | NULL          | NULL | NULL    | NULL | 4187248 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)


接下来再给表的username创建一个索引

[SQL] 纯文本查看 复制代码
1
ALTER TABLE `test1` ADD INDEX `i_name` (`username`) comment ‘‘;

   过程 会比较 卡.需要等待几秒到几十秒.取决于机器. 也可以少插入几条数据. 然后再执行刚才的两条语句

mysql> explain select username from test1 where username = ‘test‘;
+----+-------------+-------+------+---------------+--------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key    | key_len | ref   | rows | Extra                    |
+----+-------------+-------+------+---------------+--------+---------+-------+------+--------------------------+
|  1 | SIMPLE      | test1 | ref  | i_name        | i_name | 7       | const |    1 | Using where; Using index |
+----+-------------+-------+------+---------------+--------+---------+-------+------+--------------------------+
1 row in set (0.00 sec)

mysql> explain select username from test1;
+----+-------------+-------+-------+---------------+--------+---------+------+---------+-------------+
| id | select_type | table | type  | possible_keys | key    | key_len | ref  | rows    | Extra       |
+----+-------------+-------+-------+---------------+--------+---------+------+---------+-------------+
|  1 | SIMPLE      | test1 | index | NULL          | i_name | 7       | NULL | 4187248 | Using index |
+----+-------------+-------+-------+---------------+--------+---------+------+---------+-------------+
1 row in set (0.00 sec)


发现 type 变了   key也变了.  extra也不一样了. 对比一下就了解情况

其中,

type=const表示通过索引一次就找到了;

key=primary的话,表示使用了主键;

type=all,表示为全表扫描;

key=null表示没用到索引。
type=ref,因为这时认为是多个匹配行,在联合查询中,一般为REF。

经过分析后就可以判断出哪些字段需要建 索引,哪些条件可以优化等. 以及条数等



system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL

















































以上是关于mysql的explain的主要内容,如果未能解决你的问题,请参考以下文章

mysql的explain

MySQL查询分析器EXPLAIN或DESC

MySQL中EXPLAIN详解

mysql之explain

[mysql] mysql explain 使用

MySQL高级知识——Explain