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

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)

Answer 3

Pretty much the same response as Adam, but I would prefer to keep the period_start as a time field so it is easier to filter further if needed:
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

Answer 4

Using oracle provided function "WIDTH_BUCKET" to accumulate continuous or fine-discrete data. The following example shows a way to create a histogram with 5 buckets and gather "COLUMN_VALUE" from 510 to 520 (so each bucket gets values of range 2). WIDTH_BUCKET will create additional id=0 and num_buckets+1 buckets for values below min and above max.
SELECT "BUCKET_ID", count(*),
CASE
WHEN "BUCKET_ID"=0 THEN -1/0F
ELSE 510+(520-510)/5*("BUCKET_ID"-1)
END "BUCKET_MIN",
CASE
WHEN "BUCKET_ID"=5+1 THEN 1/0F
ELSE 510+(520-510)/5*("BUCKET_ID")
END "BUCKET_MAX"
FROM
(
SELECT "COLUMN_VALUE",
WIDTH_BUCKET("COLUMN_VALUE", 510, 520, 5) "BUCKET_ID"
FROM "MY_TABLE"
)
group by "BUCKET_ID"
ORDER BY "BUCKET_ID";
Sample output
BUCKET_ID COUNT(*) BUCKET_MIN BUCKET_MAX
---------- ---------- ---------- ----------
0 45 -Inf 5.1E+002
1 220 5.1E+002 5.12E+002
2 189 5.12E+002 5.14E+002
3 43 5.14E+002 5.16E+002
4 3 5.16E+002 5.18E+002
In my table, there's no 518-520, so bucket with id=5 is not shown. On the other hand, there's values below min (510), so there's a bucket with id=0, gathering -inf to 510 values.