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

‪On Wed, May 6, 2020 at 6:31 PM ‫دار الآثار للنشر والتوزيع-صنعاء Dar
Alathar-Yemen‬‎ <dar_alathar@hotmail.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.

David J.