MySQL中distinct與group by之間的性能進行比較

2020-01-18 22:44:03

字體：大中小

來源：轉載

供稿：網友

最近在網上看到了一些測試，感覺不是很準確，今天親自測試了一番。得出了結論，測試過程在個人計算機上，可能不夠全面，僅供參考。

測試過程：
準備一張測試表

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 ;

建個儲存過程向表中插入10W條數據

 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

調用存儲過程插入數據

call p_test(100000);

開始測試：（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; 受影響的行: 0 時間: 0.078ms   [SQL]  select num from test_test group by num; 受影響的行: 0 時間: 0.031ms

二、num字段上創建索引

ALTER TABLE `test_test` ADD INDEX `num_index` (`num`) ;

再次查詢

 select distinct num from test_test;  select num from test_test group by num;  [SQL] select distinct num from test_test; 受影響的行: 0 時間: 0.000ms   [SQL]  select num from test_test group by num; 受影響的行: 0 時間: 0.000ms

這時候我們發現時間太小了 0.000秒都無法精確了。
我們轉到命令行下測試

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