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。
原作者:海底苍鹰
地址:http://blog.51yip.com/mysql/1105.html