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 首页 > MySQL

MySQL中distinct与group by之间的性能进行比较

51自学网 2016-09-18 http://www.51zixue.net

测试过程:
准备一张测试表 

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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    

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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

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call p_test(100000);

开始测试:(不加索引)

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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

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ALTER TABLE `test_test` ADD INDEX `num_index` (`num`) ;

再次查询   

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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

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


 
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