Re: BitmapHeapScan streaming read user and prelim refactoring

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:
> > The patch with a fix is attached. I put the test in
> > src/test/regress/sql/join.sql. It isn't the perfect location because
> > it is testing something exercisable with a join but not directly
> > related to the fact that it is a join. I also considered
> > src/test/regress/sql/select.sql, but it also isn't directly related to
> > the query being a SELECT query. If there is a better place for a test
> > of a bitmap heap scan edge case, let me know.
>
> I don't see a problem with adding this to join.sql - why wouldn't this
> count as something related to a join? Sure, it's not like this code
> matters only for joins, but if you look at join.sql that applies to a
> number of other tests (e.g. there are a couple btree tests).

I suppose it's true that other tests in this file use joins to test
other code. I guess if we limited join.sql to containing tests of join
implementation, it would be rather small. I just imagined it would be
nice if tests were grouped by what they were testing -- not how they
were testing it.

> That being said, it'd be good to explain in the comment why we're
> testing this particular plan, not just what the plan looks like.

You mean I should explain in the test comment why I included the
EXPLAIN plan output? (because it needs to contain a bitmapheapscan to
actually be testing anything)

I do have a detailed explanation in my test comment of why this
particular query exercises the code we want to test.

> > 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?

- Melanie