Re: Have I found an interval arithmetic bug?

On Fri, Apr 2, 2021 at 11:05 AM Bruce Momjian <bruce(at)momjian(dot)us> wrote:

> On Thu, Apr 1, 2021 at 09:46:58PM -0700, Bryn Llewellyn wrote:
> > Or am I misunderstanding something?
> >
> > Try this. The result of each “select” is shown as the trailing comment
> on the
> > same line. I added whitespace by hand to line up the fields.
> >
> > select interval '-1.7 years'; -- -1 years -8
> mons
> >
> > select interval '29.4 months'; -- 2 years 5
> mons 12
> > days
> >
> > select interval '-1.7 years 29.4 months'; -- 8
> mons 12
> > days << wrong
> > select interval '29.4 months -1.7 years'; -- 9
> mons 12
> > days
> >
> > select interval '-1.7 years' + interval '29.4 months'; -- 9
> mons 12
> > days
> > select interval '29.4 months' + interval '-1.7 years'; -- 9
> mons 12
> > days
> >
>

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.

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.

John