Optimal way to create a histogram/frequency distribution in Oracle?

Question

I have an events table with two columns eventkey (unique, primary-key) and createtime, which stores the creation time of the event as the number of milliseconds since Jan 1 1970 in a NUMBER column.

I would like to create a "histogram" or frequency distribution that shows me how many events were created in each hour of the past week.

Is this the best way to write such a query in Oracle, using the width_bucket() function? Is it possible to derive the number of rows that fall into each bucket using one of the other Oracle analytic functions rather than using width_bucket to determine what bucket number each row belongs to and doing a count(*) over that?

-- 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

If your createtime were a date column, this would be trivial:

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

As it is, casting the createtime column isn't too hard:

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

---

If, alternatively, you're looking for the fencepost values (for example, where do I go from the first decile (0-10%) to the next (11-20%), you'd do something like:

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

I'm unfamiliar with Oracle's date functions, but I'm pretty certain there's an equivalent way of writing this Postgres statement:

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