mysql 去重方法distinct 与 group by 性能比较

//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)

//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)