Fwd: Re: Interval "1 month" is equals to interval "30 days" - WHY?

This was supposed to go to the list. Sorry.
---------- Forwarded message ----------
From: "Michael Trausch" <mike(at)trausch(dot)us>
Date: Aug 8, 2012 10:12 AM
Subject: Re: [GENERAL] Interval "1 month" is equals to interval "30 days" -
WHY?
To: "Albe Laurenz" <laurenz(dot)albe(at)wien(dot)gv(dot)at>

There is root in accounting for this type of view of the interval. In
accounting, a month is considered to have 30 days or 4.25 weeks, and a year
is considered to have 360 days. The reason for this is that both the month
and year are easier to work with when evenly divisible. A quarter then has
90 days (30 * 3 or 360 / 4), and certain other equalities can be held true.

If you need exact date math, be prepared to spend a *lot* of time on the
problem. All exact date math operations must have a starting point, and
"exact" has different meanings depending on the application. Good luck.
On Aug 8, 2012 5:55 AM, "Albe Laurenz" <laurenz(dot)albe(at)wien(dot)gv(dot)at> wrote:

> Dmitry Koterov wrote:
> >> I've just discovered a very strange thing:
> >>
> >> SELECT '1 mon'::interval = '30 days'::interval --> TRUE???
> >>
> >> This returns TRUE (also affected when I create an unique index using
> an
> >> interval column). Why?
> >>
> >> I know that Postgres stores monthes, days and seconds in interval
> values
> >> separately. So how to make "=" to compare intervals "part-by-part"
> and not
> >> treat "1 mon" as "30 days"?
> >>
> >> P.S.
> >> Reproduced at least in 8.4 and 9.1.
>
> > ...and even worse:
> >
> > SELECT ('1 year'::interval) = ('360 days'::interval); --> TRUE :-)
> > SELECT ('1 year'::interval) = ('365 days'::interval); --> FALSE :-)
>
> Intervals are internally stored in three fields: months, days
> and microseconds. A year has 12 months.
>
> PostgreSQL converts intervals into microseconds before comparing them:
> a month is converted to 30 days, and a day is converted to 24 hours.
>
> Of course this is not always correct.
> But what should the result of
> INTERVAL '1 month' = INTERVAL '30 days'
> be? FALSE would be just as wrong.
>
> Yours,
> Laurenz Albe
>
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