MySQL中distinct和group by性能比较[转]
之前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
1CREATE TABLE `test_test` (2 `id` int(11) NOT NULL auto_increment,
3 `num` int(11) NOT NULL default '0',
4 PRIMARY KEY(`id`)5 ) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;
建个储存过程向表中插入10W条数据
01 create procedure p_test(pa int(11))02begin03
04 declare max_num int(11) default 100000;05 declare i int default 0;06 declarerand_num int;07
08 select count(id) into max_num from test_test;09
10 while i < pa do
11 if max_num < 100000then12 select cast(rand()*100 asunsigned) into rand_num;13insert into test_test(num)values(rand_num);14 end if;15 set i = i +1;16 end while;17 end
调用存储过程插入数据
1 call p_test(100000);
开始测试：(不加索引)
01select distinct num from test_test;02select num from test_test group by num;03
04[SQL] select distinct num from test_test;05 受影响的行: 0
06 时间: 0.078ms07
08[SQL]09select num from test_test group by num;10 受影响的行: 0
11 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
01select distinct num from test_test;02select num from test_test group by num;03[SQL] select distinct num from test_test;04 受影响的行: 0
05 时间: 0.000ms06
07[SQL]08select num from test_test group by num;09 受影响的行: 0
10 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
01 mysql> set profiling=1;02 mysql>select distinct(num) from test_test;03 mysql>select num from test_test group by num;04 mysql>show profiles;05 +----------+------------+----------------------------------------+
06 | Query_ID | Duration | Query |
07 +----------+------------+----------------------------------------+
08 | 1 | 0.00072550 | select distinct(num) from test_test |
09 | 2 | 0.00071650 | select num from test_test group by num |
10 +----------+------------+----------------------------------------+
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
