Re: Have I found an interval arithmetic bug? - Mailing list pgsql-hackers

On Fri, 2 Apr 2021 at 21:08, Zhihong Yu <zyu@yugabyte.com> wrote:

Hi,

I got a local build with second patch where:

yugabyte=# SELECT  interval '0.3 years' + interval '0.4 years' -
                interval '0.7 years';
 ?column?
----------
 1 mon

I think the outcome is a bit unintuitive (I would expect result close to 0).

That's not fundamentally different from this:

odyssey=> select 12 * 3/10 + 12 * 4/10 - 12 * 7/10;

 ?column? 

----------

       -1

(1 row)

odyssey=> 

And actually the result is pretty close to 0. I mean it’s less than 0.1 year.

I wonder if it might have been better if only integers had been accepted for the components? If you want 0.3 years write 0.3 * '1 year'::interval. But changing it now would be a pretty significant backwards compatibility break.

There's really no avoiding counterintuitive behaviour though. Look at this:

odyssey=> select 0.3 * '1 year'::interval + 0.4 * '1 year'::interval - 0.7 * '1 year'::interval;

     ?column?     

------------------

 -1 mons +30 days

(1 row)

odyssey=> select 0.3 * '1 year'::interval + 0.4 * '1 year'::interval - 0.7 * '1 year'::interval = '0';

 ?column? 

----------

 t

(1 row)

odyssey=> 

In other words, doing the “same” calculation but with multiplying 1 year intervals by floats to get the values to add, you end up with an interval that while not identical to 0 does compare equal to 0. So very close to 0; in fact, as close to 0 as you can get without actually being identically 0.