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

CREATE TABLE `test_test` (

`id` int(11) NOT NULL auto_increment,

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

PRIMARY KEY (`id`)

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

建个储存过程向表中插入10W条数据

create procedure p_test(pa int(11))

begin

declare max_num int(11) default 100000;

declare i int default 0;

declare rand_num int;

select count(id) into max_num from test_test;

while i < pa

do

if max_num < 100000 then

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

insert into test_test(num)values(rand_num);

end if;

set i = i +1;

end while;

end

调用存储过程插入数据

1 call p_test(100000);

开始测试：(不加索引)

select distinct num from test_test;

select num from test_test group by num;

[SQL] select distinct num from test_test;

受影响的行: 0

时间: 0.078ms

[SQL]

select num from test_test group by num;

受影响的行: 0

时间: 0.031ms

二、num字段上创建索引

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

再次查询

select distinct num from test_test;

select num from test_test group by num;

[SQL] select distinct num from test_test;

受影响的行: 0

时间: 0.000ms

[SQL]

select num from test_test group by num;

受影响的行: 0

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

mysql> set profiling=1;

mysql> select distinct(num) from test_test;

mysql> select num from test_test group by num;

mysql> show profiles;

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

| Query_ID | Duration | Query |

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

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

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

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

测试结论

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

参考推荐：