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

Of course NOT.

'1 mon' and '30 days' have different meaning. So they should not be equal.

I understand that conversion to seconds is a more or less correct way to
compare intervals with ">" and "<". But equality is not the same as
ordering (e.g. equality is typically used in JOINs and unique indices).

Now I have to use CREATE UNIQUE INDEX test ON tbl(interval_col::TEXT) and
use the same casting to TEXT in all JOINS and searches - this is very ugly.

On Wed, Aug 8, 2012 at 1:54 PM, 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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