Re: Have I found an interval arithmetic bug?

On Thu, Jul 22, 2021 at 03:17:52PM -0700, Zhihong Yu wrote:
> On Thu, Jul 22, 2021 at 2:59 PM Zhihong Yu <zyu(at)yugabyte(dot)com> wrote:
> Hi,
>
> -                   tm->tm_mon += (fval * MONTHS_PER_YEAR);
> +                   tm->tm_mon += rint(fval * MONTHS_PER_YEAR);
>
> Should the handling for year use the same check as that for month ?
>
> -                   AdjustFractDays(fval, tm, fsec, DAYS_PER_MONTH);
> +                   /* round to a full month? */
> +                   if (rint(fval * DAYS_PER_MONTH) == DAYS_PER_MONTH)
>
> Cheers 
>
> Hi,
> I guess the reason for current patch was that year to months conversion is
> accurate.

Our internal units are hours/days/seconds, so the spill _up_ from months
to years happens automatically:

SELECT INTERVAL '23.99 months';
interval
----------
2 years

> On the new test:
>
> +SELECT INTERVAL '1.16 months 01:00:00' AS "One mon 5 days one hour";
>
> 0.16 * 31 = 4.96 < 5
>
> I wonder why 5 days were chosen in the test output.

We use 30 days/month, not 31. However, I think you are missing the
changes in the patch and I am just understanding them fully now. There
are two big changes:

1. The amount of spill from months only to days
2. The _rounding_ of the result once we stop spilling at months or days

#2 is the part I think you missed.

One thing missing from my previous patch was the handling of negative
units, which is now handled properly in the attached patch:

SELECT INTERVAL '-1.99 years';
interval
----------
-2 years

SELECT INTERVAL '-1.99 months';
interval
----------
-2 mons

I ended up creating a function to handle this, which allowed me to
simplify some of the surrounding code.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EDB https://enterprisedb.com

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