對(duì)于distinct,groupby的性能。

1. 數(shù)據(jù)量非常巨大時(shí)候，比如1000萬(wàn)中有300W重復(fù)數(shù)據(jù)，這時(shí)候的distinct的效率略好于group by；
2. 對(duì)于相對(duì)重復(fù)量較小的數(shù)據(jù)量比如1000萬(wàn)中1萬(wàn)的重復(fù)量，用groupby的性能會(huì)遠(yuǎn)優(yōu)于distnct。
3. 簡(jiǎn)書(shū)上的一篇博客說(shuō)的不錯(cuò)，大家可以穿送過(guò)去看一看傳送門(mén)
   例如、找出所有員工當(dāng)前(to_date='9999-01-01')具體的薪水salary情況，對(duì)于相同的薪水只顯示一次,并按照逆序顯示
   CREATE TABLE salaries (
   emp_no int(11) NOT NULL,
   salary int(11) NOT NULL,
   from_date date NOT NULL,
   to_date date NOT NULL,
   PRIMARY KEY (emp_no,from_date));
   1.select salary
   from salaries
   where to_date='9999-01-01'
   group by salary
   order by salary desc71. select salary from salaries where to_date='9999-01-01' group by salary order by salary desc;
4. SELECT DISTINCT salary FROM salaries WHERE to_date = '9999-01-01' ORDER BY salary DESC