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MySQL中distinct與group by之間的性能進(jìn)行比較

更新時(shí)間：2015年11月09日 10:41:53 投稿：lijiao

這篇文章主要針對(duì)MySQL中distinct與group by之間的性能進(jìn)行比較，內(nèi)容比較詳細(xì)，很直觀的能看出比較結(jié)果，感興趣的小伙伴們可以參考一下

最近在網(wǎng)上看到了一些測(cè)試，感覺不是很準(zhǔn)確，今天親自測(cè)試了一番。得出了結(jié)論，測(cè)試過程在個(gè)人計(jì)算機(jī)上，可能不夠全面，僅供參考。

測(cè)試過程：

準(zhǔn)備一張測(cè)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(gè)儲(chǔ)存過程向表中插入10W條數(shù)據(jù)

 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(diào)用存儲(chǔ)過程插入數(shù)據(jù)

call p_test(100000);

開始測(cè)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(shí)間: 0.078ms
 
 [SQL] 
 select num from test_test group by num;
 受影響的行: 0
 時(shí)間: 0.031ms

二、num字段上創(chuàng)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(shí)間: 0.000ms
 
 [SQL] 
 select num from test_test group by num;
 受影響的行: 0
 時(shí)間: 0.000ms

這時(shí)候我們發(fā)現(xiàn)時(shí)間太小了 0.000秒都無法精確了。
我們轉(zhuǎn)到命令行下測(cè)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 |
 +----------+------------+----------------------------------------+