Re: BUG #16419: wrong parsing BC year in to_date() function

On Tue, 2020-05-12 at 18:09 -0700, David G. Johnston wrote:
> Redirecting to -hackers for visibility. I feel there needs to be something done here, even if just documentation (a bullet in the usage notes section - and a code comment update for the macro)
> pointing this out and not changing any behavior.
>
> David J.
>
> On Wed, May 6, 2020 at 8:12 PM David G. Johnston <david(dot)g(dot)johnston(at)gmail(dot)com> wrote:
> > ‪On Wed, May 6, 2020 at 6:31 PM ‫دار الآثار للنشر والتوزيع-صنعاء Dar Alathar-Yemen‬‎ <dar_alathar(at)hotmail(dot)com> wrote:‬
> > > Any one suppose that these functions return the same:
> > > make_date(-1,1,1)
> > > to_date('-1-01-01','yyyy-mm-dd')
> > >
> > > But make_date will give 0001-01-01 BC
> > >
> > > And to_date will give 0002-01-01 BC
> > >
> > >
> > >
> > >
> >
> > Interesting...and a fair point.
> >
> > What seems to be happening here is that to_date is trying to be helpful by doing:
> >
> > select to_date('0000','YYYY'); // 0001-01-01 BC
> >
> > It does this seemingly by subtracting one from the year, making it positive, then (I infer) appending "BC" to the result. Thus for the year "-1" it yields "0002-01-01 BC"
> >
> > make_date just chooses to reject the year 0 and treat the negative as an alternative to specifying BC
> >
> > There seems to be zero tests for to_date involving negative years, and the documentation doesn't talk of them.
> >
> > I'll let the -hackers speak up as to how they want to go about handling to_date (research how it behaves in the other database it tries to emulate and either document or possibly change the
> > behavior in v14) but do suggest that a simple explicit description of how to_date works in the presence of negative years be back-patched. A bullet in the usage notes section probably suffices:
> >
> > "If a YYYY format string captures a negative year, or 0000, it will treat it as a BC year after decreasing the value by one. So 0000 maps to 1 BC and -1 maps to 2 BC and so on."
> >
> > So, no, make_date and to_date do not agree on this point; and they do not have to. There is no way to specify "BC" in make_date function so using negative there makes sense. You can specify BC
> > in the input string for to_date and indeed that is the only supported (documented) way to do so.
> >
> >
>
>
> [and the next email]
>
> > Specifically:
> >
> > https://github.com/postgres/postgres/blob/fb544735f11480a697fcab791c058adc166be1fa/src/backend/utils/adt/formatting.c#L236
> >
> > /*
> > * There is no 0 AD. Years go from 1 BC to 1 AD, so we make it
> > * positive and map year == -1 to year zero, and shift all negative
> > * years up one. For interval years, we just return the year.
> > */
> > #define ADJUST_YEAR(year, is_interval) ((is_interval) ? (year) : ((year) <= 0 ? -((year) - 1) : (year)))
> >
> > The code comment took me a bit to process - seems like the following would be better (if its right - I don't know why interval is a pure no-op while non-interval normalizes to a positive integer).
> >
> > Years go from 1 BC to 1 AD, so we adjust the year zero, and all negative years, by shifting them away one year, We then return the positive value of the result because the caller tracks the BC/AD
> > aspect of the year separately and only deals with positive year values coming out of this macro. Intervals denote the distance away from 0 a year is so we can simply take the supplied value and
> > return it. Interval processing code expects a negative result for intervals going into BC.
> >
> > David J.

Since "to_date" is an Oracle compatibility function, here is what Oracle 18.4 has to say to that:

SQL> SELECT to_date('0000', 'YYYY') FROM dual;
SELECT to_date('0000', 'YYYY') FROM dual
*
ERROR at line 1:
ORA-01841: (full) year must be between -4713 and +9999, and not be 0

SQL> SELECT to_date('-0001', 'YYYY') FROM dual;
SELECT to_date('-0001', 'YYYY') FROM dual
*
ERROR at line 1:
ORA-01841: (full) year must be between -4713 and +9999, and not be 0

SQL> SELECT to_date('-0001', 'SYYYY') FROM dual;

TO_DATE('-0001','SYYYY
----------------------
0001-05-01 00:00:00 BC

Yours,
Laurenz Albe