Re: [PATCH] Fix overflow and underflow in regr_r2()

On Sat, 23 May 2026 at 03:42, Chengpeng Yan <chengpeng_yan(at)outlook(dot)com> wrote:
>
> Thanks for the regr_intercept.patch. The approach looks good to me.

Thanks for reviewing, and sorry for the delay getting back to you.

> 2. `dy` seems a bit hard to understand. Perhaps `offset`, as used in the
> earlier sketch, would be clearer?

[Shrug] I think dy is common enough to denote a difference in
y-values, and it seems clear enough, given the large comment above it.

> 3. Do we need to add tests for the underflow path, and perhaps for the
> Inf/NaN guard?

Yeah, I think it makes sense to include a test with underflow, since
that really can lead to a large relative error. I don't think it's
worth testing the Inf/NaN guard, since that's more about avoiding
operating on technically uninitialised variables, and I don't believe
that it actually affects the results.

I've add this test case:

SELECT regr_intercept(y, x) FROM (VALUES (-1e-131, 0), (2e-131,
3e-131)) v(x, y);

Here, directly computing Sx * Sxy / Sxx causes an underflow to zero,
while the correct result should be 1e-131. Since Sy is 3e-131, this
makes a noticeable difference to the final result (without the patch,
it returns an intercept of 1.5e-131, whereas with the patch, it
correctly returns 1e-131).

If there are no objections from the RMT, I'll push both of these (to
HEAD only) in a couple of days or so.

Regards,
Dean