From: Antonin Houska <ah@cybertec.at>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
cc: Marcos Pegoraro <marcos@f10.com.br>, Michael Banck <mbanck@gmx.net>,
    Junwang Zhao <zhjwpku@gmail.com>,
    Kirill Reshke <reshkekirill@gmail.com>,
    Pavel Stehule <pavel.stehule@gmail.com>,
    Michael Paquier <michael@paquier.xyz>,
    PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: why there is not VACUUM FULL CONCURRENTLY?
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Comments: In-reply-to Alvaro Herrera <alvherre@alvh.no-ip.org>
   message dated "Fri, 04 Apr 2025 09:33:54 +0200."
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Date: Fri, 04 Apr 2025 12:28:36 +0200
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Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:

> On 2025-Apr-01, Antonin Houska wrote:
> 
> > Besides that, it occurred to me that 0005 ("Preserve visibility
> > information of the concurrent data changes.") will probably introduce
> > significant overhead. The problem is that the table we're repacking is
> > treated like a catalog, for reorderbuffer.c to generate snapshots that
> > we need to replay UPDATE / DELETE commands on the new table.
> > 
> > contrib/test_decoding can be used to demonstrate the difference
> > between ordinary and catalog tables:
> > 
> > [.. ordinary ..]
> > Execution Time: 3521.190 ms
> > [.. catalog ..]
> > Execution Time: 6561.634 ms
> 
> Significant indeed.  Thinking about the scenarios in which I envision
> people using REPACK CONCURRENTLY (mostly, cases where very large tables
> have accumulated considerable amounts of bloat) and considering the size
> of the patch, I think the case for treating it as concurrent-safe is not
> credible, at least not at this stage -- not only because of this
> performance impact, but also because of the additional code complexity,
> which I'm really doubtful we can address at this stage.  I would suggest
> to put that patch aside for now, maybe with a doc warning that
> "repacking a table would cause visibility information to be lost"; and
> then address that aspect later on, after this feature has gone through
> some battle-hardening.

ok, I'll adjust the patch set.

-- 
Antonin Houska
Web: https://www.cybertec-postgresql.com