Re: [PATCH] Add roman support for to_number function

Thanks for the contribution.

I took a look at the patch, and it works as advertised. It's too late
for the September commitfest, but I took the liberty of registering
your patch for the November CF [1]. In the course of that, I found an
older thread proposing this feature seven years ago [2]. That patch
was returned with feedback and (as far as I can tell), was not
followed-up on by the author. You may want to review that thread for
feedback; I won't repeat it here.

On Fri, Aug 30, 2024 at 12:22 AM Hunaid Sohail <hunaidpgml(at)gmail(dot)com> wrote:
>While looking at formatting.c file, I noticed a TODO about "add support for roman number to standard number conversion" (https://github.com/postgres/postgres/blob/master/src/backend/utils/adt/formatting.c#L52)

Your patch should also remove the TODO =)

> - Is it okay for the function to handle Roman numerals in a case-insensitive way? (e.g., 'XIV', 'xiv', and 'Xiv' are all seen as 14).

The patch in the thread I linked also took a case-insensitive
approach. I did not see any objections to that, and it seems
reasonable to me as well.

> - How should we handle Roman numerals with leading or trailing spaces, like ' XIV' or 'MC '? Should we trim the spaces, or would it be better to throw an error in such cases?

I thought we could reference existing to_number behavior here, but
after playing with it a bit, I'm not really sure what that is:

-- single leading space
maciek=# select to_number(' 1', '9');
to_number
-----------
1
(1 row)

-- two leading spaces
maciek=# select to_number(' 1', '9');
ERROR: invalid input syntax for type numeric: " "
-- two leading spaces and template pattern with a decimal
maciek=# select to_number(' 1', '9D9');
to_number
-----------
1
(1 row)

Separately, I also noticed some unusual Roman representations work
with your patch:

postgres=# select to_number('viv', 'RN');
to_number
-----------
9
(1 row)

Is this expected? In contrast, some somewhat common alternative
representations don't work:

postgres=# select to_number('iiii', 'RN');
ERROR: invalid roman numeral

I know this is expected, but is this the behavior we want? If so, we
probably want to reject the former case, too. If not, maybe that one
is okay, too.

I know I've probably offered more questions than answers, but I hope
finding the old thread here is useful.

Thanks,
Maciek

[1]: https://commitfest.postgresql.org/50/5233/
[2]: https://www.postgresql.org/message-id/flat/CAGMVOduAJ9wKqJXBYnmFmEetKxapJxrG3afUwpbOZ6n_dWaUnA%40mail.gmail.com