MySQL中distinct和group by性能比较

矫情吗；* 2022-05-23 20:40 181阅读 0赞

### 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

![复制代码][copycode.gif]

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

![复制代码][copycode.gif]

调用存储过程插入数据

1    call p_test(100000);

**开始测试：（不加索引）**

![复制代码][copycode.gif]

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

![复制代码][copycode.gif]

![2012091214591962.jpg][]

二、num字段上创建索引

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

再次查询

![复制代码][copycode.gif]

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

![复制代码][copycode.gif]

![2012091215005085.jpg][]

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

![复制代码][copycode.gif]

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    +----------+------------+----------------------------------------+

![复制代码][copycode.gif]

![2012091215015392.jpg][]

**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

转载自[https://www.cnblogs.com/zox2011/archive/2012/09/12/2681797.html][https_www.cnblogs.com_zox2011_archive_2012_09_12_2681797.html]

[copycode.gif]: /images/20220524/b2a8551423ea45b080e0ad9ce99da359.png
[2012091214591962.jpg]: /images/20220524/232f435593e045d190adbfda87a96293.png
[2012091215005085.jpg]: /images/20220524/4f807237db514ac785129537ca768ad8.png
[2012091215015392.jpg]: /images/20220524/109af409dd684458ac32addf38ac94a0.png
[https_www.cnblogs.com_zox2011_archive_2012_09_12_2681797.html]: https://www.cnblogs.com/zox2011/archive/2012/09/12/2681797.html