Re: Have I found an interval arithmetic bug? - Mailing list pgsql-hackers

Hi,

bq. My new code returns 0.2 months for this, not zero

Can you clarify (the output below that was 2 mons, not 0.2) ?

Thanks

On Fri, Apr 2, 2021 at 4:58 PM Bruce Momjian <bruce@momjian.us> wrote:

On Fri, Apr  2, 2021 at 02:00:03PM -0700, John W Higgins wrote:
> On Fri, Apr 2, 2021 at 11:05 AM Bruce Momjian <bruce@momjian.us> wrote:
> While maybe there is an argument to fixing the negative/positive rounding issue
> - there is no way this gets solved without breaking the current implementation
>
> select interval '0.3 years' + interval '0.4 years' - interval '0.7 years' +
> interval '0.1 years' should not equal 0 but it certainly does.

My new code returns 0.2 months for this, not zero:

        SELECT  interval '0.3 years' + interval '0.4 years' -
                interval '0.7 years' + interval '0.1 years';
         ?column?
        ----------
         2 mons

which is also wrong since:

        SELECT interval '0.1 years';
         interval
        ----------
         1 mon

> Unless we take the concept of 0.3 years = 3 months and move to something along
> the lines of 
>
> 1 year = 360 days
> 1 month = 30 days 
>
> so therefore 
>
> 0.3 years = 360 days * 0.3 = 108 days = 3 months 18 days 
> 0.4 years = 360 days * 0.4 = 144 days = 4 months 24 days
> 0.7 years = 360 days * 0.7 = 252 days = 8 months 12 days
>
> Then, and only if we don't go to any more than tenths of a year, does the math
> work. Probably this should resolve down to seconds and then work backwards -
> but unless we're looking at breaking the entire way it currently resolves
> things - I don't think this is of much value.
>
> Doing math on intervals is like doing math on rounded numbers - there is always
> going to be a pile of issues because the level of precision just is not good
> enough.

I think the big question is what units do people want with fractional
values.  I have posted a follow-up email that spills only for one unit,
which I think is the best approach.

--
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  If only the physical world exists, free will is an illusion.