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

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 |
 +----------+------------+----------------------------------------+

分析：
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。