Re: [SQL] Interval subtracting

Mark Dilger wrote:
> Tom Lane wrote:
> > "Milen A. Radev" <milen(at)radev(dot)net> writes:
> >
> >>Milorad Poluga ????????????:
> >>
> >>>>SELECT '10 years 1 mons 1 days'::interval - '9 years 10 mons 15 days'::interval
> >>>>?column?
> >>>>---------------
> >>>>3 mons -14 days
> >>>>
> >>>>Why not '2 mons 16 days' ?
> >
> >
> >>Please read the last paragraph in section 8.5.1.4 of the manual
> >>(http://www.postgresql.org/docs/8.1/static/datatype-datetime.html#AEN4775)
> >>. It mentions the functions named "justify_days" and "justify_hours"
> >>that could do what you need.
> >
> >
> > justify_days doesn't currently do anything with this result --- it
> > thinks its charter is only to reduce day components that are >= 30 days.
> > However, I think a good case could be made that it should normalize
> > negative days too; that is, the invariant on its result should be
> > 0 <= days < 30, not merely days < 30. Similarly for justify_hours.
> > Comments anyone? Patch anyone?
>
> Sure, if nobody objects to this change I can write the patch.

Good question. Should we restrict days to 0 - 30 or -30 - 30? The
current system does the later:

test=> select justify_days('-45 days');
justify_days
------------------
-1 mons -15 days
(1 row)

test=> select justify_days('1 month -45 days');
justify_days
--------------
-15 days
(1 row)

test=> select justify_days('1 month -15 days');
justify_days
----------------
1 mon -15 days
(1 row)

Should we be adjusting the last one? I am unsure. Comments?

--
Bruce Momjian http://candle.pha.pa.us
SRA OSS, Inc. http://www.sraoss.com

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