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
1 CREATE TABLE `test_test` (2`id` int(11) NOT NULL auto_increment,3`num` int(11) NOT NULL default '0',4PRIMARY 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)) 02begin 03 04declare max_num int(11) default 100000; 05declare i int default 0; 06declare rand_num int; 07 08select count(id) into max_num from test_test; 09 10while i < pa do 11 if max_num < 100000 then 12 select cast(rand()*100 as unsigned) into rand_num; 13 insert into test_test(num)values(rand_num); 14 end if; 15 set i = i +1; 16end while; 17end
调用存储过程插入数据
1 call p_test(100000);
开始测试:(不加索引)
01 select distinct num from test_test; 02 select num from test_test group by num; 0304 [SQL] select distinct num from test_test;05 受影响的行: 006 时间: 0.078ms0708 [SQL] 09 select num from test_test group by num;10 受影响的行: 011 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
01 select distinct num from test_test; 02 select num from test_test group by num; 03 [SQL] select distinct num from test_test;04 受影响的行: 005 时间: 0.000ms0607 [SQL] 08 select num from test_test group by num;09 受影响的行: 010 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
转载自/zox/archive//09/12/2681797.html