Στις Tuesday 13 May 2008 11:22:28 ο/η Scott Marlowe έγραψε:
> On Tue, May 13, 2008 at 12:56 AM, Achilleas Mantzios
> <achill@matrix.gatewaynet.com> wrote:
> > ���� Monday 12 May 2008 18:09:11 �/� Tom Lane ������:
> >> Achilleas Mantzios <achill@matrix.gatewaynet.com> writes:
> >> > ���� Monday 12 May 2008 17:32:39 �/� Julius Tuskenis ������:
> >> >> do you know why division of intervals was not implemented? As I see it -
> >> >> there should be an easy way to do so. '1 hour 30 minutes' / '1 hour' =
> >> >> 1.5 .... Seems straight forward...
> >> >>
> >> > No idea why. However as you pose it,it makes sense.
> >>
> >> No, it doesn't really, because intervals contain multiple components.
> >> What would you define as the result of
> >> '1 month 1 hour' / '1 day'
> >> bearing in mind that the number of days per month is not fixed, and
> >> neither is the number of hours per day?
> >>
> >
> > SELECT extract(epoch from '1 month'::interval)/(60*60*24) as "Days in 1 month interval";
> > Days in 1 month interval
> > --------------------------
> > 30
> > (1 row)
> > So it seems that the arbitary assumption of (1 month=30 days, and 1 day = 24 hours) has already been made
> > in the extract function.
>
> True. But that's only because it doesn't have a date to work against.
> If you run:
>
> select '2007-02-01 12:00:00'::timestamp + '1 month'::interval;
> you get: 2007-03-01 12:00:00
>
> If you run:
> select '2007-03-01 12:00:00'::timestamp + '1 month'::interval;
> you get: 2007-04-01 12:00:00
>
> Then, if we run:
> select ('2007-03-01 12:00:00'::timestamp + '1 month'::interval) -
> '2007-03-01 12:00:00'::timestamp;
> we get: 31 days
>
> But if we run:
> select ('2007-02-01 12:00:00'::timestamp + '1 month'::interval) -
> '2007-02-01 12:00:00'::timestamp;
> we get: 28 days
>
> So, I'm not sure how many days a month has.
>
> But your point is valid that given no date to compare to, an interval
> of 1 month gets translated to 30 days. I can also see a lot of people
> showing up doing fancy interval math THEN wondering why it changes
> when you put a timestamp in the middle of it.
>
Thanx for this well written explanation!
--
Achilleas Mantzios