今天无意中听到有同事在讨论，distinct和group by谁的速度会更快一点，意件不一，其实我也不知道那个好，下午有时间做了一下测试。

1，测试前的准备

//准备一张测试表

mysql> 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 ;

Query OK, 0 rows affected (0.05 sec)

mysql> delimiter || //改变mysql命令结束符为||

//建个储存过程向表中插入10W条数据

mysql> 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||

Query OK, 0 rows affected (0.00 sec)

mysql> call p_test(100000)||

Query OK, 1 row affected (5.66 sec)

mysql> delimiter ;//改变mysql命令结束符为;

mysql> select count(id) from test_test; //数据都进去了

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

| count(id) |

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

| 100000 |

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

1 row in set (0.00 sec)

mysql> show variables like "%pro%"; //查看一下，记录执行的profiling是不是开启动了，默认是不开启的

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

| Variable_name | Value |

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

| profiling | OFF |

| profiling_history_size | 15 |

| protocol_version | 10 |

| slave_compressed_protocol | OFF |

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

4 rows in set (0.00 sec)

mysql> set profiling=1; //开启

Query OK, 0 rows affected (0.00 sec)

2，测试

//做了4组测试

mysql> select distinct(num) from test_test;

mysql> select num from test_test group by num;

mysql> show profiles; //查看结果

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

| Query_ID | Duration | Query |

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

| 1 | 0.07298225 | select distinct(num) from test_test |

| 2 | 0.07319975 | select num from test_test group by num |

| 3 | 0.07313525 | select num from test_test group by num |

| 4 | 0.07317725 | select distinct(num) from test_test |

| 5 | 0.07275200 | select distinct(num) from test_test |

| 6 | 0.07298600 | select num from test_test group by num |

| 7 | 0.07500700 | select num from test_test group by num |

| 8 | 0.07331325 | select distinct(num) from test_test |

| 9 | 0.57831575 | create index num_index on test_test (num) | //在这儿的时候，我加了索引

| 10 | 0.00243550 | select distinct(num) from test_test |

| 11 | 0.00121975 | select num from test_test group by num |

| 12 | 0.00116550 | select distinct(num) from test_test |

| 13 | 0.00107650 | select num from test_test group by num |

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

13 rows in set (0.00 sec)

上面的1-8是4组数据，并且是没有加索引的，从中我们可以看出，distinct比group by 会好一点点

10-13是2组数据，是加了索引以后的，从中我们可以看出，group by 比distinct 会好一点点

一般情况，数据量比较大的表，关联字段都会加索引的，，并且加索引后检索时间只有以前的六分之一左右。