Re: Have I found an interval arithmetic bug?

On Wed, 21 Jul 2021 at 03:48, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
>
> this example now gives me concern:
>
> SELECT INTERVAL '1.06 months 1 hour';
> interval
> -----------------------
> 1 mon 2 days 01:00:00
>
> Notice that it rounds the '1.06 months' to '1 mon 2 days', rather than
> spilling to hours/minutes/seconds, even though hours is already
> specified. I don't see a better way to handle this than the current
> code already does, but it is something odd.

Hmm, looking at this whole thread, I have to say that I prefer the old
behaviour of spilling down to lower units.

For example, with this patch:

SELECT '0.5 weeks'::interval;
interval
----------
4 days

which I don't think is really an improvement. My expectation is that
half a week is 3.5 days, and I prefer what it used to return, namely
'3 days 12:00:00'.

It's true that that leads to odd-looking results when the field value
has lots of fractional digits, but that was at least explainable, and
followed the documentation.

Looking for a general principle to follow, how about this -- the
result of specifying a fractional value should be the same as
multiplying an interval of 1 unit by that value. In other words,
'1.8594 months'::interval should be the same as '1 month'::interval *
1.8594. (Actually, it probably can't easily be made exactly the same
in all cases, due to differences in the floating point computations in
the two cases, and rounding errors, but it's hopefully not far off,
unlike the results obtained by not spilling down to lower units on
input.)

The cases that are broken in master, in my opinion, are the larger
units (year and above), which don't propagate down in the same way as
fractional months and below. So, for example, '0.7 years' should be
8.4 months (with the conversion factor of 1 year = 12 months), giving
'8 months 12 days', which is what '1 year'::interval * 0.7 produces.
Sure, there are arguably more accurate ways of computing that.
However, that's the result obtained using the documented conversion
factors, so it's justifiable in those terms.

It's worth noting another case that is broken in master:

SELECT '1.7 decades'::interval;
interval
------------------
16 years 11 mons

which is surely not what anyone would expect. The current patch fixes
this, but it would also be fixed by handling the fractional digits for
these units in the same way as for smaller units. There was an earlier
patch doing that, I think, though I didn't test it.

Regards,
Dean