Re: HOT chain validation in verify_heapam()

Hi,

On 2022-11-15 11:36:21 -0500, Robert Haas wrote:
> On Mon, Nov 14, 2022 at 5:02 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > It seems like we should do a bit more validation within a chain of
> > tuples. E.g. that no live tuple can follow an !DidCommit xmin?
>
> I think this check is already present in stronger form. If we see a
> !DidCommit xmin, the xmin of the next tuple in the chain not only can't be
> committed, but had better be the same.

As I think I mentioned before, I don't think the "better be the same" aspect
is correct, think subxacts. E.g.

off 0: xmin: top, xmax: child_1
off 1: xmin: child_1, xmax: invalid

If top hasn't committed yet, the current logic afaict will warn about this
situation, no? And I don't think we can generally the subxid parent at this
point, unfortunately (might have truncated subtrans).

Different aspect: Is it ok that we use TransactionIdDidCommit() without a
preceding IsInProgress() check?

I do think there's some potential for additional checks that don't run into
the above issue, e.g. checking that no in-progress xids follow an explicitly
aborted xact, that a committed xid can't follow an uncommitted xid etc.

Greetings,

Andres Freund