mysql distinct跟group by性能

mysql distinct和group by性能  

1，测试前的准备

1. //准备一张测试表
2. mysql> CREATE TABLE `test_test` (
3. ->   `id` int(11) NOT NULL auto_increment,
4. ->   `num` int(11) NOT NULL default '0',
5. ->   PRIMARY KEY  (`id`)
6. -> ) ENGINE=MyISAM  DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;
7. Query OK, 0 rows affected (0.05 sec)
8. mysql> delimiter ||  //改变mysql命令结束符为||
9. //建个储存过程向表中插入10W条数据
10. mysql> create procedure p_test(pa int(11))
11. -> begin
12. ->
13. ->  declare max_num int(11) default 100000;
14. ->  declare i int default 0;
15. ->  declare rand_num int;
16. ->
17. ->  select count(id) into max_num from test_test;
18. ->
19. ->  while i < pa do
20. ->          if max_num < 100000 then
21. ->                  select cast(rand()*100 as unsigned) into rand_num;
22. ->                  insert into test_test(num)values(rand_num);
23. ->          end if;
24. ->          set i = i +1;
25. ->  end while;
26. -> end||
27. Query OK, 0 rows affected (0.00 sec)
28. mysql> call p_test(100000)||
29. Query OK, 1 row affected (5.66 sec)
30. mysql> delimiter ;//改变mysql命令结束符为;
31. mysql> select count(id) from test_test;  //数据都进去了
32. +-----------+
33. | count(id) |
34. +-----------+
35. |    100000 |
36. +-----------+
37. 1 row in set (0.00 sec)
38. 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
39. +---------------------------+-------+
40. | Variable_name             | Value |
41. +---------------------------+-------+
42. | profiling                 | OFF   |
43. | profiling_history_size    | 15    |
44. | protocol_version          | 10    |
45. | slave_compressed_protocol | OFF   |
46. +---------------------------+-------+
47. 4 rows in set (0.00 sec)
48. mysql> set profiling=1;           //开启
49. Query OK, 0 rows affected (0.00 sec)

//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

1. //做了4组测试
2. mysql> select distinct(num) from test_test;
3. mysql> select num from test_test group by num;
4. mysql> show profiles;    //查看结果
5. +----------+------------+-------------------------------------------+
6. | Query_ID | Duration   | Query                                     |
7. +----------+------------+-------------------------------------------+
8. |        1 | 0.07298225 | select distinct(num) from test_test       |
9. |        2 | 0.07319975 | select num from test_test group by num    |
10. |        3 | 0.07313525 | select num from test_test group by num    |
11. |        4 | 0.07317725 | select distinct(num) from test_test       |
12. |        5 | 0.07275200 | select distinct(num) from test_test       |
13. |        6 | 0.07298600 | select num from test_test group by num    |
14. |        7 | 0.07500700 | select num from test_test group by num    |
15. |        8 | 0.07331325 | select distinct(num) from test_test       |
16. |        9 | 0.57831575 | create index num_index on test_test (num) |  //在这儿的时候，我加了索引
17. |       10 | 0.00243550 | select distinct(num) from test_test       |
18. |       11 | 0.00121975 | select num from test_test group by num    |
19. |       12 | 0.00116550 | select distinct(num) from test_test       |
20. |       13 | 0.00107650 | select num from test_test group by num    |
21. +----------+------------+-------------------------------------------+
22. 13 rows in set (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)

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。

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