Re: pgsql: Fix numeric_mul() overflow due to too many digits after decimal

[ moving to pghackers for wider visibility ]

Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com> writes:
> On Sat, 10 Jul 2021 at 16:01, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> In general, I'm disturbed that we just threw away the previous
>> promise that numeric multiplication results were exact. That
>> seems like a pretty fundamental property --- which is stated
>> in so many words in the manual, btw --- and I'm not sure I want
>> to give it up.

> Perhaps we should amend the statement about numeric multiplication to
> say that it's exact within the limits of the numeric type's supported
> scale, which we also document in the manual as 16383.
> That seems a lot better than throwing an overflow error for a result
> that isn't very big, which limits what's possible with numeric
> multiplication to much less than 16383 digits.

TBH, I don't agree. I think this is strictly worse than what we
did before, and we should just revert it. It's no longer possible
to reason about what numeric multiplication will do. I think
throwing an error if we can't represent the result exactly is a
preferable behavior. If you don't want exact results, use float8.

regards, tom lane