query to return hourly snapshot

I orginally sent this email to the [novice] list but did not get any response.

I am look for help in developing a query that will return the nearest process record that was
logged at or after each hour in a day (i.e. hourly snapshot).


Below are typical sample data. Also, I've included a query used to return the average of the
process value for each hour to give an example for the format that I am trying to achieve.

Thanks for the help.

Regards,

Richard

select * from process limit 10;
tstamp | process
---------------------+---------
2005-10-25 21:10:41 | 3.56513
2005-10-25 21:10:42 | 3.56503
2005-10-25 21:10:43 | 3.56494
2005-10-25 21:10:44 | 3.56484
2005-10-25 21:10:45 | 3.56475
2005-10-25 21:10:46 | 3.56465
2005-10-25 21:10:47 | 3.56455
2005-10-25 21:10:48 | 3.56446
2005-10-25 21:10:49 | 3.56436
2005-10-25 21:10:50 | 3.56427
(10 rows)

select avg(process),
date_trunc('hour',tstamp) as date
from process
where date_trunc('day', tstamp) = '2005-10-26'
group by date_trunc('hour', tstamp)
order by date_trunc('hour', tstamp);

avg | date
------------------+---------------------
9.79195118032606 | 2005-10-26 05:00:00
10.0249767947376 | 2005-10-26 06:00:00
8.88596018049452 | 2005-10-26 07:00:00
7.95090951088542 | 2005-10-26 08:00:00
8.10741349776586 | 2005-10-26 09:00:00
7.30079822791947 | 2005-10-26 10:00:00
7.10586501293712 | 2005-10-26 11:00:00
8.15196838166979 | 2005-10-26 12:00:00
8.26183129151662 | 2005-10-26 13:00:00
8.95141531440947 | 2005-10-26 14:00:00
10.562882253329 | 2005-10-26 15:00:00
10.8634908244445 | 2005-10-26 16:00:00
11.4077104069976 | 2005-10-26 17:00:00
12.4702264580744 | 2005-10-26 18:00:00
11.9155618293134 | 2005-10-26 19:00:00
11.5622152555012 | 2005-10-26 20:00:00
11.6527367563489 | 2005-10-26 21:00:00
10.3170960432442 | 2005-10-26 22:00:00
9.56747980806563 | 2005-10-26 23:00:00
(19 rows)

On Tue, Apr 04, 2006 at 05:58:26PM -0700, Richard Broersma Jr wrote:
> I am look for help in developing a query that will return the nearest
> process record that was logged at or after each hour in a day (i.e.
> hourly snapshot).

Are you looking for something like this?

SELECT p.process, date_trunc('hour', p.tstamp) AS hour
FROM process AS p
JOIN (
SELECT date_trunc('hour', tstamp), min(tstamp)
FROM process
WHERE date_trunc('day', tstamp) = '2005-10-26'
GROUP BY date_trunc('hour', tstamp)
) AS s ON s.min = p.tstamp
ORDER BY hour;

Or, using PostgreSQL's non-standard DISTINCT ON clause:

SELECT DISTINCT ON (date_trunc('hour', tstamp))
process, date_trunc('hour', tstamp) AS hour
FROM process
WHERE date_trunc('day', tstamp) = '2005-10-26'
ORDER BY date_trunc('hour', tstamp), tstamp;

--
Michael Fuhr

Yes! Thanks you very much!

--- Michael Fuhr <mike(at)fuhr(dot)org> wrote:

> On Tue, Apr 04, 2006 at 05:58:26PM -0700, Richard Broersma Jr wrote:
> > I am look for help in developing a query that will return the nearest
> > process record that was logged at or after each hour in a day (i.e.
> > hourly snapshot).
>
> Are you looking for something like this?
>
> SELECT p.process, date_trunc('hour', p.tstamp) AS hour
> FROM process AS p
> JOIN (
> SELECT date_trunc('hour', tstamp), min(tstamp)
> FROM process
> WHERE date_trunc('day', tstamp) = '2005-10-26'
> GROUP BY date_trunc('hour', tstamp)
> ) AS s ON s.min = p.tstamp
> ORDER BY hour;
>
> Or, using PostgreSQL's non-standard DISTINCT ON clause:
>
> SELECT DISTINCT ON (date_trunc('hour', tstamp))
> process, date_trunc('hour', tstamp) AS hour
> FROM process
> WHERE date_trunc('day', tstamp) = '2005-10-26'
> ORDER BY date_trunc('hour', tstamp), tstamp;
>
> --
> Michael Fuhr
>
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