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

From: Andres Freund <andres(at)anarazel(dot)de>
To: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Cc: "Wood, Dan" <hexpert(at)amazon(dot)com>, Peter Geoghegan <pg(at)bowt(dot)ie>, pgsql-committers(at)postgresql(dot)org, "Wong, Yi Wen" <yiwong(at)amazon(dot)com>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: [HACKERS] pgsql: Fix freezing of a dead HOT-updated tuple
Date: 2017-11-03 14:53:30
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On 2017-11-02 06:05:51 -0700, Andres Freund wrote:
> Hi,
> On 2017-11-02 13:49:47 +0100, Alvaro Herrera wrote:
> > Andres Freund wrote:
> > > I think the problem is on the pruning, rather than the freezing side. We
> > > can't freeze a tuple if it has an alive predecessor - rather than
> > > weakining this, we should be fixing the pruning to not have the alive
> > > predecessor.
> >
> > I gave a look at HTSV back then, but I didn't find what the right tweak
> > was, but then I only tried changing the return value to DEAD and
> > DELETE_IN_PROGRESS; the thought of selecting DEAD or RECENTLY_DEAD based
> > on OldestXmin didn't occur to me ... I was thinking that the fact that
> > there were live lockers meant that the tuple could not be removed,
> > obviously failing to notice that the subsequent versions of the tuple
> > would be good enough.
> I'll try to write up a commit based on that idea. I think there's some
> comment work needed too, Robert and I were both confused by a few
> things.
> I'm unfortunately travelling atm - it's evening here, and I'll flying
> back to the US all Saturday. I'm fairly sure I'll be able to come up
> with a decent patch tomorrow, but I'll need review and testing by
> others.

Here's that patch. I've stared at this some, and Robert did too. Robert
mentioned that the commit message might need some polish and I'm not
100% sure about the error message texts yet.

I'm not yet convinced that the new elog in vacuumlazy can never trigger
- but I also don't think we want to actually freeze the tuple in that

Staring at the vacuumlazy hunk I think I might have found a related bug:
heap_update_tuple() just copies the old xmax to the new tuple's xmax if
a multixact and still running. It does so without verifying liveliness
of members. Isn't that buggy? Consider what happens if we have three
blocks: 1 has free space, two is being vacuumed and is locked, three is
full and has a tuple that's key share locked by a live tuple and is
updated by a dead xmax from before the xmin horizon. In that case afaict
the multi will be copied from the third page to the first one. Which is
quite bad, because vacuum already processed it, and we'll set
relfrozenxid accordingly. I hope I'm missing something here?


Andres Freund

Attachment Content-Type Size
0001-Fix-pruning-of-locked-and-updated-tuples.patch text/x-diff 7.9 KB

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