Re: BUG #17949: Adding an index introduces serialisation anomalies.

> On Thu, Jun 22, 2023 at 10:02:19PM +1200, Thomas Munro wrote:
> On Wed, Jun 21, 2023 at 9:04 PM Dmitry Dolgov <9erthalion6@gmail.com> wrote:
> > Now the reading transaction actually does PredicateLockPage on the
> > metabuffer inside scanPendingInsert, but strangely enough it doesn't
> > lock anything because the SerializationNeededForRead condition is false.
> > I'm trying to verify if it's somehow a part of the issue, or something
> > is broken on my side.
>
> Maybe you were confused by the presence of non-SSI transactions in the
> repro (eg the transaction that sets up the index)?

Yeah, sort of. Need to optimize the way how I consume the logs.

> To answer my own earlier question, the conflict-in check for posting
> trees is hidden in getFindLeafPage(..., true, ...).
>
> I spent some more time trying to grok this today.  FTR it reproduces
> faster without the extra tuple that repro I posted inserts after
> TRUNCATE (the point of that was to find out whether it was an
> empty-to-non-empty transition).  I still don't know what's wrong but I
> am beginning to suspect the "fast" code.  It seems as though, under
> high concurrency, we sometimes don't scan a recently inserted
> (invisible to our snapshot, but needed for SSI checks) tuple, but I
> don't yet know why.

Yep, it's definitely something in the "fast" path. Testing the same, but
with an index having (fastupdate=off) works just fine for me.