MYSQL-實現ORACLE- row_number() over(partition by ) 分組排序功能優化

今天看了篇帖子，被幾個地方轉載，但是性能不太好，因為不能評論，所以把優化思路寫在這里。

1.確定需求: 根據部門來分組,顯示各員工在部門里按薪水排名名次.

2.來創建實例數據:

drop table if exists heyf_t10;

create table heyf_t10 (empid int ,deptid int ,salary decimal(10,2) );

insert into heyf_t10 values

(1,10,5500.00),

(2,10,4500.00),

(3,20,1900.00),

(4,20,4800.00),

(5,40,6500.00),

(6,40,14500.00),

(7,40,44500.00),

(8,50,6500.00),

(9,50,7500.00);

3. http://www.kaishixue.com/mysql/14.html 帖子中SQL的實現

select empid,deptid,salary,rank

from

(select heyf_tmp.empid, heyf_tmp.deptid, heyf_tmp.salary, @rownum:=@rownum+1,

if(@pdept=heyf_tmp.deptid,@rank:=@rank+1,@rank:=1) as rank,

@pdept:=heyf_tmp.deptid

from

(select empid,deptid,salary from heyf_t10 order by deptid asc ,salary desc ) heyf_tmp,

(select @rownum:=0, @pdept:=null,@rank:=0) a ) result;

執行計划如下

idselect_typetabletypepossible_keyskeykey_lenrefrowsfilteredExtra
1PRIMARY<derived2>ALLNULLNULLNULLNULL9100.00
2DERIVED<derived4>systemNULLNULLNULLNULL1100.00
2DERIVED<derived3>ALLNULLNULLNULLNULL9100.00
4DERIVEDNULLNULLNULLNULLNULLNULLNULLNULLNo tables used
3DERIVEDheyf_t10ALLNULLNULLNULLNULL9100.00Using filesort

如果大數據量直接就掛了。換個思路來寫

select h.`empid`,h.`deptid`,h.`salary`,count(*) as rank
from heyf_t10 as h
left outer join heyf_t10 as r
on h.deptid = r.deptid
and h.`salary`<= r.`salary`
group by h.`empid`,h.`deptid`,h.`salary`
order by h.deptid, h.salary desc;

idselect_typetabletypepossible_keyskeykey_lenrefrowsfilteredExtra
1SIMPLEhALLNULLNULLNULLNULL9100.00Using temporary; Using filesort
1SIMPLErALLNULLNULLNULLNULL9100.00

需要優化的時候建立個索引優化掉filesort，很直觀，效率也提高了很多。有時候不能太趕需求，遇到復雜的SQL還是讓DBA來優化一下。

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