Re: BitmapHeapScan streaming read user and prelim refactoring

On Wed, Apr 24, 2024 at 4:46 PM Melanie Plageman
<melanieplageman(at)gmail(dot)com> wrote:
>
> On Tue, Apr 23, 2024 at 6:43 PM Tomas Vondra
> <tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
> >
> > On 4/23/24 18:05, Melanie Plageman wrote:
> > > One other note: there is some concurrency effect in the parallel
> > > schedule group containing "join" where you won't trip the assert if
> > > all the tests in that group in the parallel schedule are run. But, if
> > > you would like to verify that the test exercises the correct code,
> > > just reduce the group containing "join".
> > >
> >
> > That is ... interesting. Doesn't that mean that most test runs won't
> > actually detect the problem? That would make the test a bit useless.
>
> Yes, I should really have thought about it more. After further
> investigation, I found that the reason it doesn't trip the assert when
> the join test is run concurrently with other tests is that the SELECT
> query doesn't use the skip fetch optimization because the VACUUM
> doesn't set the pages all visible in the VM. In this case, it's
> because the tuples' xmins are not before VacuumCutoffs->OldestXmin
> (which is derived from GetOldestNonRemovableTransactionId()).
>
> After thinking about it more, I suppose we can't add a test that
> relies on the relation being all visible in the VM in a group in the
> parallel schedule. I'm not sure this edge case is important enough to
> merit its own group or an isolation test. What do you think?

Andres rightly pointed out to me off-list that if I just used a temp
table, the table would only be visible to the testing backend anyway.
I've done that in the attached v2. Now the test is deterministic.

- Melanie