Re: Fortify float4 and float8 regression tests by ordering test results

Hi, Tom!

On Tue, 22 Apr 2025 at 21:13, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Pavel Borisov <pashkin.elfe@gmail.com> writes:
> > On Tue, 22 Apr 2025 at 20:22, Alexander Korotkov <aekorotkov@gmail.com> wrote:
> >> On Tue, Apr 22, 2025 at 7:20 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >>> Alexander Korotkov <aekorotkov@gmail.com> writes:
> >>>> I'd like to add that float4.out not only assumes that insert-ordering is
> >>>> preserved (this could be more-or-less portable between table AMs).  It also
> >>>> assumes the way UPDATE moves updated rows.  That seems quite
> >>>> heap-specific.  You can see in the following fragment, updated rows jump to
> >>>> the bottom.
>
> >>> I'd be willing to consider a policy that we don't want to depend on
> >>> exactly where UPDATE moves rows to.  The proposed patch is not that,
> >>> however.
>
> >> OK, that makes sense for me.
>
> > Thanks for this input!
> > This was my first intention to fix only the test that was affected by
> > UPDATE-order specifics, broke when runnung on an extension AM.
> > Maybe I was too liberal and added ORDER BY's more than needed.
> > I definitely agree to the proposal.
>
> On further reflection, this policy makes plenty of sense because even
> heapam is not all that stable about row order after UPDATE.  With
> tables large enough to draw the attention of autovacuum, we've had
> to use ORDER BY or other techniques to stabilize the results, because
> the timing of background autovacuums affected where rows got put.
> These tests are different only because the tables are small enough
> and the updates few enough that autovacuum doesn't get involved.
> I think it's reasonable that at a project-policy level we should
> not consider that an excuse.  For example, a change in autovacuum's
> rules could probably break these tests even with heapam.
>
> On the other hand, as I mentioned earlier, a table AM that doesn't
> reproduce original insertion order would break a whole lot more
> tests than these.  So that's a bridge I'd rather not cross,
> at least not without a lot of consideration.
>
> BTW, you forgot to update expected/float4-misrounded-input.out.
Added in v3. Thanks for a mention!

Regards,
Pavel Borisov