From: Achilleas Mantzios <achill@matrix.gatewaynet.com>
Organization: Dynacom Tankers Mgmt
To: "Scott Marlowe" <scott.marlowe@gmail.com>
Subject: Re: how to get a number of seconds in some interval ?
Date: Tue, 13 May 2008 11:41:00 +0300
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Cc: pgsql-admin@postgresql.org, "Tom Lane" <tgl@sss.pgh.pa.us>,
	"Julius Tuskenis" <julius@nsoft.lt>
References: <482849B0.6050103@nsoft.lt>
	<200805130956.51872.achill@matrix.gatewaynet.com>
	<dcc563d10805130122s7ef074bfra86ee43978c5c65f@mail.gmail.com>
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=CE=A3=CF=84=CE=B9=CF=82 Tuesday 13 May 2008 11:22:28 =CE=BF/=CE=B7 Scott M=
arlowe =CE=AD=CE=B3=CF=81=CE=B1=CF=88=CE=B5:
> On Tue, May 13, 2008 at 12:56 AM, Achilleas Mantzios
> <achill@matrix.gatewaynet.com> wrote:
> > =D3=F4=E9=F2 Monday 12 May 2008 18:09:11 =EF/=E7 Tom Lane =DD=E3=F1=E1=
=F8=E5:
> >> Achilleas Mantzios <achill@matrix.gatewaynet.com> writes:
> >> > =D3=F4=E9=F2 Monday 12 May 2008 17:32:39 =EF/=E7 Julius Tuskenis =DD=
=E3=F1=E1=F8=E5:
> >> >> 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=
' =3D
> >> >> 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=3D30 days, and 1 d=
ay =3D 24 hours) has already been made
> > in the extract function.
>=20
> True.  But that's only because it doesn't have a date to work against.
>  If you run:
>=20
> select '2007-02-01 12:00:00'::timestamp + '1 month'::interval;
> you get: 2007-03-01 12:00:00
>=20
> If you run:
> select '2007-03-01 12:00:00'::timestamp + '1 month'::interval;
> you get:  2007-04-01 12:00:00
>=20
> 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
>=20
> 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
>=20
> So, I'm not sure how many days a month has.
>=20
> 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.
>=20

Thanx for this well written explanation!

=2D-=20
Achilleas Mantzios