Re: how to get a number of seconds in some interval ?

On Tue, May 13, 2008 at 12:56 AM, Achilleas Mantzios
<achill(at)matrix(dot)gatewaynet(dot)com> wrote:
> Στις Monday 12 May 2008 18:09:11 ο/η Tom Lane έγραψε:
>> Achilleas Mantzios <achill(at)matrix(dot)gatewaynet(dot)com> writes:
>> > Στις Monday 12 May 2008 17:32:39 ο/η Julius Tuskenis έγραψε:
>> >> 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.