MySQL中distinct与group by之间的性能进行比较

2020-01-18 22:44:03

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最近在网上看到了一些测试，感觉不是很准确，今天亲自测试了一番。得出了结论，测试过程在个人计算机上，可能不够全面，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

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

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