Re: BitmapHeapScan streaming read user and prelim refactoring

On Thu, May 2, 2024 at 5:37 PM Tomas Vondra
<tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>
>
>
> On 4/24/24 22:46, Melanie Plageman 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:
> >>> 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)
> >
>
> No, I meant that the comment before the test describes a couple
> requirements the plan needs to meet (no need to process all inner
> tuples, bitmapscan eligible for skip_fetch on outer side, ...), but it
> does not explain why we're testing that plan.
>
> I could get to that by doing git-blame to see what commit added this
> code, and then read the linked discussion. Perhaps that's enough, but
> maybe the comment could say something like "verify we properly discard
> tuples on rescans" or something like that?

Attached is v3. I didn't use your exact language because the test
wouldn't actually verify that we properly discard the tuples. Whether
or not the empty tuples are all emitted, it just resets the counter to
0. I decided to go with "exercise" instead.

- Melanie