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Re: Time problem again?

From: Bjørn T Johansen <btj(at)havleik(dot)no>
To: Richard Huxton <dev(at)archonet(dot)com>
Cc: Postgres General <pgsql-general(at)postgresql(dot)org>
Subject: Re: Time problem again?
Date: 2003-09-30 09:54:37
Message-ID: 1064915676.20427.18.camel@dt-btj.dagbladet.no (view raw or flat)
Thread:
Lists: pgsql-general
Oki, I will check it out....
Thx! :)


BTJ

On Tue, 2003-09-30 at 11:24, Richard Huxton wrote:
> On Tuesday 30 September 2003 09:50, Bjørn T Johansen wrote:
> > On Tue, 2003-09-30 at 10:42, Richard Huxton wrote:
> > > So - your rule is something like:
> > >
> > > For some block of times...
> > >   IF max(my_time) - min(my_time) > 12 hours
> > >   THEN sort "through midnight"
> > >   ELSE sort "naturally"
> > >
> > > Which is what Tom said (I was wondering where he got his 12 hours from).
> > > Have I got that right?
> >
> > Yes, that sounds about right.. :)
> > But how do I code this in an Select statement?
> 
> Well, I'd write a function (notice the double-quoting):
> 
> -- sort_times(TARGET-TIME, DIFFERENCE)
> --  Takes a target time and the difference max(t)-min(t) in its group
> --  Returns a timestamp you can sort on
> --
> CREATE FUNCTION sort_times(time, interval) 
> RETURNS timestamptz AS '
> SELECT
>   CASE
>     WHEN $2 > ''12:00''::interval AND $1<=''12:00:00''::time 
>       THEN ''1970-01-02 00:00:00+00''::timestamptz + $1
>     ELSE ''1970-01-01 00:00:00+00''::timestamptz + $1
>   END
> ' LANGUAGE 'SQL' IMMUTABLE;
> 
> Then you have the wrong way:
> 
> SELECT
>   id, grp, ts
> FROM
>   timetest
> ORDER BY
>   grp, ts
> ;
> 
>  id | grp |    ts
> ----+-----+----------
>   1 | a   | 11:00:00
>   2 | a   | 14:00:00
>   3 | a   | 17:00:00
>   4 | a   | 20:00:00
>   7 | b   | 01:00:00 ***
>   8 | b   | 04:00:00 *** Oops - these are not 
>   5 | b   | 20:00:00 *** what we wanted
>   6 | b   | 22:00:00 ***
>   9 | c   | 03:00:00
>  10 | c   | 06:00:00
>  11 | c   | 08:00:00
> (11 rows)
> 
> And the right way:
> SELECT
>   t.id, t.grp, t.ts
> FROM
>   timetest t,
>   (SELECT grp, max(ts)-min(ts) as tdiff FROM timetest GROUP BY grp) AS diffs
> WHERE
>   t.grp = diffs.grp
> ORDER BY
>   t.grp, sort_times(t.ts, diffs.tdiff)
> ;
> 
>  id | grp |    ts
> ----+-----+----------
>   1 | a   | 11:00:00
>   2 | a   | 14:00:00
>   3 | a   | 17:00:00
>   4 | a   | 20:00:00
>   5 | b   | 20:00:00 ***
>   6 | b   | 22:00:00 *** Ah - better!
>   7 | b   | 01:00:00 ***
>   8 | b   | 04:00:00 ***
>   9 | c   | 03:00:00
>  10 | c   | 06:00:00
>  11 | c   | 08:00:00
> (11 rows)
> 
> I'm not sure how PG will optimise the correctly sorted one - you'll have to 
> try it on your real data and see.


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