MySQL中distinct与group by之间的性能进行比较

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MySQL中distinct与group by之间的性能进行比较

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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

1	`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

1	`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 |
								+----------+------------+----------------------------------------+

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