Re: New vacuum option to do only freezing

From: Andres Freund <andres(at)anarazel(dot)de>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>, Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>, "Bossart, Nathan" <bossartn(at)amazon(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: New vacuum option to do only freezing
Date: 2019-05-02 14:28:30
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On 2019-05-02 10:20:25 -0400, Robert Haas wrote:
> On Tue, Apr 16, 2019 at 12:44 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > So after thinking about this a bit more ...
> > 2. Act as if the tuple were still live, just as would've happened if the
> > state didn't change till just after we looked instead of just before.
> >
> > Option #2 is a lot simpler and safer, and can be implemented as I
> > suggested earlier, assuming we're all good with the assumption that
> > heap_prepare_freeze_tuple isn't going to do anything bad.
> After studying this more carefully, I agree. You and Andres and
> Alvaro are all correct, and I'm plain wrong. Thanks for explaining.
> I have committed a patch that changes the logic as per your
> suggestion, and also removes nleft_dead_tuples and nleft_dead_itemids.

It'd be good if somebody could make a pass over the safety mechanisms in
heap_prepare_freeze_tuple(). I added them at some point, after a data
corrupting bug related to freezing, but they really were more intended
as a secondary layer of defense, rather than the primary one. My
understanding is still that we assume that we never should reach
heap_prepare_freeze_tuple() for something that is below the horizon,
even after this change, right?


Andres Freund

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