MySQL中distinct與group by之間的性能進行比較

2024-07-24 12:45:29

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