Re: Have I found an interval arithmetic bug?
| От | Isaac Morland |
|---|---|
| Тема | Re: Have I found an interval arithmetic bug? |
| Дата | |
| Msg-id | CAMsGm5errRXqOOt2r-uSDYqn301s_tQMQdJHLPmTXcc+EOLEfQ@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Have I found an interval arithmetic bug? (Zhihong Yu <zyu@yugabyte.com>) |
| Ответы |
Re: Have I found an interval arithmetic bug?
|
| Список | 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 monI 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.
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