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

From Zhihong Yu
Subject Re: Have I found an interval arithmetic bug?
Date
Msg-id CALNJ-vQkNfeM--i_L9XSpXsP_sj=UYFnAtF+u2ZhGUSe0PW6Cw@mail.gmail.com
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In response to Re: Have I found an interval arithmetic bug?  (Zhihong Yu <zyu@yugabyte.com>)
Responses Re: Have I found an interval arithmetic bug?  (Bruce Momjian <bruce@momjian.us>)
List pgsql-hackers


On Sun, Apr 11, 2021 at 4:33 PM Zhihong Yu <zyu@yugabyte.com> wrote:


On Sun, Apr 11, 2021 at 12:57 PM Zhihong Yu <zyu@yugabyte.com> wrote:


On Thu, Apr 8, 2021 at 10:24 AM Bruce Momjian <bruce@momjian.us> wrote:
On Mon, Apr  5, 2021 at 02:01:58PM -0400, Bruce Momjian wrote:
> On Mon, Apr  5, 2021 at 11:33:10AM -0500, Justin Pryzby wrote:
> Well, bug or not, we are not going to change back branches for this, and
> if you want a larger discussion, it will have to wait for PG 15.
>
> > > https://www.postgresql.org/docs/current/datatype-datetime.html#DATATYPE-INTERVAL-INPUT
> > > « …field values can have fractional parts; for example '1.5 week' or '01:02:03.45'. Such input is converted to the appropriate number of months, days, and seconds for storage. When this would result in a fractional number of months or days, the fraction is added to the lower-order fields using the conversion factors 1 month = 30 days and 1 day = 24 hours. For example, '1.5 month' becomes 1 month and 15 days. Only seconds will ever be shown as fractional on output. »
>
> I see that.  What is not clear here is how far we flow down.  I was
> looking at adding documentation or regression tests for that, but was
> unsure.  I adjusted the docs slightly in the attached patch.

Here is an updated patch, which will be for PG 15.  It updates the
documentation to state:

        The fractional parts are used to compute appropriate values for the next
        lower-order internal fields (months, days, seconds).

It removes the flow from fractional months/weeks to
hours-minutes-seconds, and adds missing rounding for fractional
computations.

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

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


+1 to this patch. 

Bryn reminded me, off list, about the flowing down from fractional day after the patch.

Before Bruce confirms the removal of the flowing down from fractional day, I withhold my previous +1.

Bryn would respond with more details.

Cheers

Among previous examples given by Bryn, the following produces correct result based on Bruce's patch.

# select interval '-1.7 years 29.4 months';
    interval
----------------
 9 mons 12 days
(1 row)

Cheers

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