这篇文章主要介绍了MySQL如何实现百分位数计算，具有一定借鉴价值，感兴趣的朋友可以参考下，希望大家阅读完这篇文章之后大有收获，下面让小编带着大家一起了解一下。



创建试验数据,5天每天一百万随机数据,总共500w数据

create table nums(id int not null primary key);
delimiter $$
begin
truncate table nums;
while s*2<=cnt do
set s=s*2;
end $$

call pFastCreateNums(2000000);

drop table if exists t ;
create table t(
query_time date,
ts float,
key(query_time,ts)
);


insert into t select '2018-07-01',round(100000*rand(),2) from nums where id<=1000000;
insert into t select '2018-07-02',round(100000*rand(),2) from nums where id<=1000000;
insert into t select '2018-07-03',round(100000*rand(),2) from nums where id<=1000000;
insert into t select '2018-07-04',round(100000*rand(),2) from nums where id<=1000000;
insert into t select '2018-07-05',round(100000*rand(),2) from nums where id<=1000000;

首先，修正上文的SQL，增加精度,因为在大数据量下,会有显著的误差。


select query_time,v,ts

from (

select t6.query_time,t6.ts,v,seq,

case when @gid=concat(seq,'#',query_time) then @rn:=@rn+1 when @gid:=concat(seq,'#',query_time) then @rn:=1 end s

from (

select query_time,ts,rn,percent,v,v-percent d,seq from (

select t2.query_time,ts,rn,round(rn/total,10) percent from (

select query_time,ts,

case when @gid=query_time then @rn:=@rn+1 when @gid:=query_time then @rn:=1 end rn

from (

select * from t ,(select @gid:='',@rn:=0) vars order by query_time,ts

) t1

) t2 inner join (

select query_time,count(*) total from t group by query_time

) t3 on(t2.query_time=t3.query_time)

) t4 ,

(select 0.71 v,1 seq union all select 0.81,2 union all select 0.91,3) t5

) t6 where d>=0 order by query_time,v,d

) t7 where s=1 order by query_time,seq ;


在ssd环境下,上文的SQL运行时长和结果如下.

148.813 s


前文这个SQL的计算结果是非常精确的
但是计算时间和 采样点数量 有巨大关系. 假如原始数据是100w，三个百分位数的采样,则数据扩张到300w;4个百分位数的采样,则数据扩张到400w.这是因为使用笛卡尔积扩张了数据的缘故.

优化版本:

select query_time,d,max(ts) ts from (

select t2.query_time,ts,rn,round(rn/total,10) percent,

case

when 0.71>=round(rn/total,10) then 0.71

when 0.81>=round(rn/total,10) then 0.81

when 0.91>=round(rn/total,10) then 0.91

end d

from (

select query_time,ts,

case when @gid=query_time then @rn:=@rn+1 when @gid:=query_time then @rn:=1 end rn

from (

select * from t ,(select @gid:='',@rn:=0) vars order by query_time,ts

) t1

) t2 inner join (

select query_time,count(*) total from t group by query_time

) t3 on(t2.query_time=t3.query_time)

) t6

where d is not null

group by query_time,d


结果:

用时:
33.922 秒

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