Re: [COMMITTERS] pgsql: Fix freezing of a dead HOT-updated tuple

Wood, Dan wrote:
> I’m unclear on what is being repro’d in 9.6. Are you getting the
> duplicate rows problem or just the reindex problem? Are you testing
> with asserts enabled(I’m not)?

I was seeing just the reindex problem. I don't see any more dups.

But I've tried to reproduce it afresh now, and let it run for a long
time and nothing happened. Maybe I made a mistake last week and
ran an unfixed version. I don't see any more problems now.

> If you are getting the dup rows consider the code in the block in
> heapam.c that starts with the comment “replace multi by update xid”.
>
> When I repro this I find that MultiXactIdGetUpdateXid() returns 0.
> There is an updater in the multixact array however the status is
> MultiXactStatusForNoKeyUpdate and not MultiXactStatusNoKeyUpdate. I
> assume this is a preliminary status before the following row in the
> hot chain has it’s multixact set to NoKeyUpdate.

Yes, the "For" version is the locker version rather than the actual
update. That lock is acquired by EvalPlanQual locking the row just
before doing the update. I think GetUpdateXid has no reason to return
such an Xid, since it's not an update.

> Since a 0 is returned this does precede cutoff_xid and
> TransactionIdDidCommit(0) will return false. This ends up aborting
> the multixact on the row even though the real xid is committed. This
> sets XMAX to 0 and that row becomes visible as one of the dups.
> Interestingly the real xid of the updater is 122944 and the cutoff_xid
> is 122945.

I haven't seen this effect. Please keep us updated if you're able to
verify corruption this way.

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Álvaro Herrera https://www.2ndQuadrant.com/
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