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：
1 CREATE 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 ;
01 create procedure p_test(pa int(11)) 02 begin 03 04 declare max_num int(11) default 100000; 05 declare i int default 0; 06 declare rand_num int; 07 08 select count(id) into max_num from test_test; 09 10 while 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; 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）
01 select distinct num from test_test; 02 select 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: 0.078ms 07 08 [SQL] 09 select 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
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: 0 05 时间: 0.000ms 06 07 [SQL] 08 select 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。
43倍。
再来对比 ：group by
group by
转载自https://www.cnblogs.com/zox2011/archive/2012/09/12/2681797.html