MySQL中distinct和group by性能比较

MySQL中distinct和group by性能比较[转]

之前看了网上的一些测试，感觉不是很准确，今天亲自测试了一番。得出了结论(仅在个人计算机上测试，可能不全面，仅供参考)

测试过程：

准备一张测试表 

1    CREATE TABLE `test_test` (

2      `id` int(11) NOT NULL auto_increment,

3      `num` int(11) NOT NULL default '0',

4      PRIMARY KEY  (`id`)

5     ) ENGINE=MyISAM  DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

建个储存过程向表中插入10W条数据 

01    create procedure p_test(pa int(11)) 

02     begin 

03       

04      declare max_num int(11) default 100000; 

05      declare i int default 0; 

06      declare rand_num int; 

07       

08      select count(id) into max_num from test_test; 

09       

10      while i < pa do 

11              if max_num < 100000 then 

12                      select cast(rand()*100 as unsigned) into rand_num; 

13                      insert into test_test(num)values(rand_num); 

14              end if; 

15              set i = i +1; 

16      end while; 

17     end

调用存储过程插入数据

1    call p_test(100000);

开始测试：（不加索引）

01    select distinct num from test_test; 

02    select num from test_test group by num; 

03     

04    [SQL] select distinct num from test_test;

05    受影响的行: 0

06    时间: 0.078ms

07     

08    [SQL]  

09    select num from test_test group by num;

10    受影响的行: 0

11    时间: 0.031ms

二、num字段上创建索引

 

1    ALTER TABLE `test_test` ADD INDEX `num_index` (`num`) ;

 

再次查询

01    select distinct num from test_test; 

02    select num from test_test group by num; 

03    [SQL] select distinct num from test_test;

04    受影响的行: 0

05    时间: 0.000ms

06     

07    [SQL]  

08    select num from test_test group by num;

09    受影响的行: 0

10    时间: 0.000ms

这时候我们发现时间太小了 0.000秒都无法精确了。

 

我们转到命令行下 测试

01    mysql> set profiling=1;

02    mysql> select distinct(num) from test_test; 

03    mysql> select num from test_test group by num;

04    mysql> show profiles;

05    +----------+------------+----------------------------------------+

06    | Query_ID | Duration   | Query                                  |

07    +----------+------------+----------------------------------------+

08    |        1 | 0.00072550 | select distinct(num) from test_test    |

09    |        2 | 0.00071650 | select num from test_test group by num |

10    +----------+------------+----------------------------------------+

 

加了索引之后 distinct 比没加索引的 distinct 快了 107倍。

 

加了索引之后 group by 比没加索引的 group by 快了 43倍。

 

再来对比 ：distinct  和 group by

 

不管是加不加索引 group by 都比 distinct 快。因此使用的时候建议选 group by