在Oracle中创建直方图/频率分布的最佳方法？

Question

我有一个events表，其中有两列eventkey（唯一主键）和createtime，它将事件的创建时间存储为NUMBER列中1970年1月1日以来的毫秒数。

我想创建一个“直方图”或频率分布，显示在过去一周的每个小时内创建了多少个事件。

这是使用width_bucket()函数在Oracle中编写此类查询的最佳方法吗？是否有可能使用其他Oracle分析函数之一推导出落入每个存储桶的行数，而不是使用width_bucket来确定每行所属的存储分区数量，并在此基础上执行count(*)？

-- 1305504000000 = 5/16/2011 12:00am GMT 
-- 1306108800000 = 5/23/2011 12:00am GMT 
select 
timestamp '1970-01-01 00:00:00' + numtodsinterval((1305504000000/1000 + (bucket * 60 * 60)), 'second') period_start, 
numevents 
from (
    select bucket, count(*) as events from (
    select eventkey, createtime, 
    width_bucket(createtime, 1305504000000, 1306108800000, 24 * 7) bucket 
    from events 
    where createtime between 1305504000000 and 1306108800000 
) group by bucket 
) 
order by period_start 

Answer 1

如果您createtime是一个日期列，这将是微不足道的：

SELECT TO_CHAR(CREATE_TIME, 'DAY:HH24'), COUNT(*) 
    FROM EVENTS 
GROUP BY TO_CHAR(CREATE_TIME, 'DAY:HH24'); 

因为它是，铸造createtime列不是太难：

select TO_CHAR( 
     TO_DATE('19700101', 'YYYYMMDD') + createtime/86400000), 
     'DAY:HH24') AS BUCKET, COUNT(*) 
    FROM EVENTS 
    WHERE createtime between 1305504000000 and 1306108800000 
group by TO_CHAR( 
     TO_DATE('19700101', 'YYYYMMDD') + createtime/86400000), 
     'DAY:HH24') 
order by 1 

---

如果您正在寻找fencepost值（例如，我从哪里开始第一个十分位数（0-10％ ）到下（11-20％），你会做这样的事情：

select min(createtime) over (partition by decile) as decile_start, 
     max(createtime) over (partition by decile) as decile_end, 
     decile 
    from (select createtime, 
       ntile (10) over (order by createtime asc) as decile 
      from events 
     where createtime between 1305504000000 and 1306108800000 
     ) 

Answer 2

我不熟悉Oracle的日期函数，但我敢肯定有写这篇声明的Postgres的等效方式：

select date_trunc('hour', stamp), count(*) 
from your_data 
group by date_trunc('hour', stamp) 
order by date_trunc('hour', stamp) 

Answer 3

差不多亚当相同的反应，但我宁愿如果需要保持period_start为时间字段，以便更容易进一步筛选：

with 
events as 
(
    select rownum eventkey, round(dbms_random.value(1305504000000, 1306108800000)) createtime 
    from dual 
    connect by level <= 1000 
) 
select 
    trunc(timestamp '1970-01-01 00:00:00' + numtodsinterval(createtime/1000, 'second'), 'HH') period_start, 
    count(*) numevents 
from 
    events 
where 
    createtime between 1305504000000 and 1306108800000 
group by 
    trunc(timestamp '1970-01-01 00:00:00' + numtodsinterval(createtime/1000, 'second'), 'HH') 
order by 
    period_start