SQL:快速累积频率查询(Postgres)
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10-10-2019 - |
题
我希望从数据库中获取累积频率数据。我创建了一个简单的临时表,其中包含我们看到的所有唯一状态更新计数,以及具有该状态更新量的用户数量。
Table "pg_temp_4.statuses_count_tmp"
Column | Type | Modifiers
----------------+---------+-----------
statuses_count | integer |
frequency | bigint |
Indexes:
"statuses_count_idx" UNIQUE, btree (statuses_count)
我当前的查询是:
select statuses_count, frequency/(select * from total_statuses)::float, (select sum(frequency)/(select * from total_statuses)::float AS percentage from statuses_count_tmp WHERE statuses_count <= SCT.statuses_count) AS cumulative_percent FROM statuses_count_tmp AS SCT ORDER BY statuses_count DESC;
但这需要一段时间,查询数量很快就会增长。因此,有了我约有50,000行,我正在寻找要阅读的50k阶乘行。坐在这里看查询磨碎,我希望有一个更好的解决方案。
希望得到这样的东西:
0 0.26975161 0.26975161
1 0.15306534 0.42281695
2 0.05513516 0.47795211
3 0.03050646 0.50845857
4 0.02064444 0.52910301
解决方案
假设您具有PostgreSQL 8.4或更高版本,则应使用窗口功能解决。我猜那 total_statuses
是沿着线路的视图或温度表 select sum(frequency) from statuses_count_tmp
?我在此处将其写为CTE,应该在声明期间仅计算一次结果:
with total_statuses as (select sum(frequency) from statuses_count_tmp)
select statuses_count,
frequency / (select * from total_statuses) as frequency,
sum(frequency) over(order by statuses_count)
/ (select * from total_statuses) as cumulative_frequency
from statuses_count_tmp
没有8.4的窗口功能,您最好的选择就是仅仅是迭代处理数据:
create type cumulative_sum_type as ( statuses_count int, frequency numeric, cumulative_frequency numeric );
create or replace function cumulative_sum() returns setof cumulative_sum_type strict stable language plpgsql as $$
declare
running_total bigint := 0;
total bigint;
data_in record;
data_out cumulative_sum_type;
begin
select sum(frequency) into total from statuses_count_tmp;
for data_in in select statuses_count, frequency from statuses_count_tmp order by statuses_count
loop
data_out.statuses_count := data_in.statuses_count;
running_total := running_total + data_in.frequency;
data_out.frequency = data_in.frequency::numeric / total;
data_out.cumulative_frequency = running_total::numeric / total;
return next data_out;
end loop;
end;
$$;
select * from cumulative_sum();
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