Re: Have I found an interval arithmetic bug?

On Fri, Jul 30, 2021 at 12:49:34PM -0400, Tom Lane wrote:
> Bruce Momjian <bruce(at)momjian(dot)us> writes:
> > Now that I think of it, I will just remove the word "rounded" from the
> > back branch docs so we are technically breaking the documented API less
> > in PG 15.
>
> I think your first idea was better. Not documenting the behavior
> doesn't make this not an API change; it just makes it harder for
> people to understand what changed.

OK. However, I thought we were more worried about changing documented
APIs than undocumented ones. Anyway, I will do as you suggested.

> The doc patch itself is not exactly fine:
>
> + Field values can have fractional parts; for example, <literal>'1.5
> + weeks'</literal> or <literal>'01:02:03.45'</literal>. However,
>
> I think "some field values", as it was worded previously, was better.
> If you try to write 01.5:02:03, that is not going to be interpreted
> as 1.5 hours. (Hmm, I get something that seems quite insane:
>
> regression=# select '01.5:02:03'::interval;
> interval
> ----------------
> 1 day 14:03:00
> (1 row)
>
> I wonder what it thinks it's doing there.)

It thinks 01.5:02:03 is Days:Hours;Minute, so I think all fields can use
fractions:

SELECT interval '1.5 minutes';
interval
----------
00:01:30

> This is wrong:
>
> + because interval internally stores only three integer units (months,
> + days, seconds), fractional units must be spilled to smaller units.
>
> s/seconds/microseconds/ is probably enough to fix that.

OK, there were a few place that said "seconds" so I fixed those too.

> + For example, because months are approximated to equal 30 days,
> + fractional values of units greater than months is rounded to be the
> + nearest integer number of months. Fractional units of months or less
> + are computed to be an integer number of days and seconds, assuming
> + 24 hours per day. For example, <literal>'1.5 months'</literal>
> + becomes <literal>1 month 15 days</literal>.
>
> This entire passage is vague, and grammatically shaky too. Perhaps
> more like
>
> Fractional parts of units larger than months are rounded to the
> nearest integer number of months; for example '1.5 years'
> becomes '1 year 6 mons'. Fractional parts of months are rounded
> to the nearest integer number of days, using the assumption that
> one month equals 30 days; for example '1.5 months'

The newest patch actually doesn't work as explained above --- fractional
months now continue to spill to microseconds. I think you are looking
at a previous version.

> becomes '1 mon 15 days'. Fractional parts of days and weeks
> are converted to microseconds, using the assumption that one day
> equals 24 hours.

Uh, fractional weeks can be integer days.

> On output, the months field is shown as an appropriate number of
> years and months; the days field is shown as-is; the microseconds
> field is converted to hours, minutes, and possibly-fractional
> seconds.

Here is an updated patch that includes some of your ideas.

--
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.