mysql distinct和group by谁更好
时间:2014-07-06 17:09 来源:linux.it.net.cn 作者:IT网
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
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//准备一张测试表
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mysql> CREATE TABLE `test_test` (
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-> `id` int(11) NOT NULL auto_increment,
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-> `num` int(11) NOT NULL default '0',
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-> PRIMARY KEY (`id`)
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-> ) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;
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Query OK, 0 rows affected (0.05 sec)
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mysql> delimiter || //改变mysql命令结束符为||
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//建个储存过程向表中插入10W条数据
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mysql> create procedure p_test(pa int(11))
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-> begin
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->
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-> declare max_num int(11) default 100000;
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-> declare i int default 0;
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-> declare rand_num int;
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->
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-> select count(id) into max_num from test_test;
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->
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-> while i < pa do
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-> if max_num < 100000 then
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-> select cast(rand()*100 as unsigned) into rand_num;
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-> insert into test_test(num)values(rand_num);
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-> end if;
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-> set i = i +1;
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-> end while;
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-> end||
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Query OK, 0 rows affected (0.00 sec)
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mysql> call p_test(100000)||
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Query OK, 1 row affected (5.66 sec)
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mysql> delimiter ;//改变mysql命令结束符为;
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mysql> select count(id) from test_test; //数据都进去了
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+-----------+
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| count(id) |
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+-----------+
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| 100000 |
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+-----------+
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1 row in set (0.00 sec)
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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
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+---------------------------+-------+
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| Variable_name | Value |
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+---------------------------+-------+
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| profiling | OFF |
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| profiling_history_size | 15 |
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| protocol_version | 10 |
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| slave_compressed_protocol | OFF |
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+---------------------------+-------+
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4 rows in set (0.00 sec)
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mysql> set profiling=1; //开启
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Query OK, 0 rows affected (0.00 sec)
2，测试
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//做了4组测试
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mysql> select distinct(num) from test_test;
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mysql> select num from test_test group by num;
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mysql> show profiles; //查看结果
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+----------+------------+-------------------------------------------+
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| Query_ID | Duration | Query |
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+----------+------------+-------------------------------------------+
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| 1 | 0.07298225 | select distinct(num) from test_test |
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| 2 | 0.07319975 | select num from test_test group by num |
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| 3 | 0.07313525 | select num from test_test group by num |
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| 4 | 0.07317725 | select distinct(num) from test_test |
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| 5 | 0.07275200 | select distinct(num) from test_test |
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| 6 | 0.07298600 | select num from test_test group by num |
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| 7 | 0.07500700 | select num from test_test group by num |
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| 8 | 0.07331325 | select distinct(num) from test_test |
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| 9 | 0.57831575 | create index num_index on test_test (num) | //在这儿的时候，我加了索引
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| 10 | 0.00243550 | select distinct(num) from test_test |
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| 11 | 0.00121975 | select num from test_test group by num |
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| 12 | 0.00116550 | select distinct(num) from test_test |
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| 13 | 0.00107650 | select num from test_test group by num |
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+----------+------------+-------------------------------------------+
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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。
(责任编辑：IT)
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
2，测试
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。 (责任编辑：IT) |