Re: factorial of negative numbers

On Tue, 16 Jun 2020 at 08:48, Peter Eisentraut <
peter(dot)eisentraut(at)2ndquadrant(dot)com> wrote:

> On 2020-06-15 13:15, Ashutosh Bapat wrote:
> > Here are some comments
> > I see below in the .out but there's not corresponding SQL in .sql.
> > +SELECT factorial(-4);
> > + factorial
> > +-----------
> > + 1
> > +(1 row)
> > +
> >
> > Should we also add -4! to cover both function as well as the operator?
>
> I will add that. I wasn't actually sure about the precedence of these
> operators, so it is interesting as a regression test for that as well.
>
> +1.

> > + if (num < 0)
> > + ereport(ERROR,
> > + (errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
> >
> > This looks more of ERRCODE_FEATURE_NOT_SUPPORTED esp. since factorial
> > of negative numbers is defined but we are not supporting it. I looked
> > at some other usages of this error code. All of them are really are
> > out of range value errors.
>
> The proposed error message says this is undefined. If we use an error
> code that says it's not implemented, then the message should also
> reflect that.

Yes. BTW, OUT_OF_RANGE is not exactly "undefined" either. I searched for an
error code for "UNDEFINED" result but didn't find any.

> But that would in turn open an invitation for someone to
> implement it, and I'm not sure we want that.

It will be more complex code, so difficult to implement but why do we
prevent why not.

> It could go either way,
> but we should be clear on what we want.
>
>
Divison by zero is really undefined, 12345678 * 12345678 (just some
numbers) is out of range of say int4, but factorial of a negative number
has some meaning and is defined but PostgreSQL does not support it.

--
Best Wishes,
Ashutosh