Date: Tue, 16 Apr 2019 10:54:34 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Masahiko Sawada <sawada.mshk@gmail.com>,
	Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>,
	"Bossart, Nathan" <bossartn@amazon.com>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: New vacuum option to do only freezing
Message-ID: <20190416145434.GA23796@alvherre.pgsql>
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On 2019-Apr-16, Robert Haas wrote:

> On Mon, Apr 15, 2019 at 9:07 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > > I'm not sure that's correct.  If you do that, it'll end up in the
> > > non-tupgone case, which might try to freeze a tuple that should've
> > > been removed.  Or am I confused?
> >
> > If we're failing to remove it, and it's below the desired freeze
> > horizon, then we'd darn well better freeze it instead, no?
> 
> I don't know that that's safe.  IIRC, the freeze code doesn't cope
> nicely with being given a tuple that actually ought to have been
> deleted.  It'll just freeze it anyway, which is obviously bad.

Umm, but if we fail to freeze it, we'll leave a tuple around that's
below the relfrozenxid for the table, causing later pg_commit to be
truncated and error messages saying that the tuple cannot be read, no?

I remember that for a similar case in multixact-land, what we do is
generate a fresh multixact that carries the members that are still alive
(ie. those that cause the multixact to be kept rather than remove it),
and relabel the tuple with that one.  So the old multixact can be
removed safely.  Obviously we cannot do that for XIDs, but I do wonder
what can possibly cause a tuple to be unfreezable yet the XID to be
below the freeze horizon.  Surely if the transaction is that old, we
should have complained about it, and generated a freeze horizon that was
even older?

> Unless this has been changed since I last looked at it.

I don't think so.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services