Re: Have I found an interval arithmetic bug?

On Wed, Jul 21, 2021 at 06:39:26PM -0700, Bryn Llewellyn wrote:
> Your statement
>
>
>     “months-to-days conversion is almost always an approximation, while the
>     days to seconds conversion is almost always accurate.”
>
>
> is misleading. Any conversion like these (and also the “spill up” conversions
> that the justify_hours(), justify_days(), and justify_interval() built-in
> functions bring) are semantically dangerous because of the different rules for
> adding a pure months, a pure days, or a pure seconds interval to a timestamptz
> value.

We are trying to get the most reasonable output for fractional values
--- I stand by my statements.

> Unless you avoid mixed interval values, then it’s so hard (even though it is
> possible) to predict the outcomes of interval arithmetic. Rather, all you get
> is emergent behavior that I fail to see can be relied upon in deliberately
> designed application code. Here’s a telling example:

The point is that we will get unusual values, so we should do the best
we can.

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

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

-                   tm->tm_mon += (fval * MONTHS_PER_YEAR);
-                   AdjustFractDays(fval, tm, fsec, DAYS_PER_MONTH);
+                   /* round to a full month? */