看了网上的一些测试,感觉不是很准确,今天亲自测试了一番。得出了结论

仅在个人计算机上测试,可能不全面,仅供参考

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

performance-comparison-of-mysql-distinct-and-group-by-01

 

二、num字段上创建索引

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

performance-comparison-of-mysql-distinct-and-group-by-02

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

performance-comparison-of-mysql-distinct-and-group-by-03

 

测试结论

加了索引之后 distinct 比没加索引的 distinct 快了 107倍。

加了索引之后 group by 比没加索引的 group by 快了 43倍。

再来对比 :distinct  和 group by

不管是加不加索引 group by 都比 distinct 快。

因此使用的时候建议选 group by

 

参考推荐

group by与distinct效率分析及优化