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From: Andres Freund <[email protected]>
To: Álvaro Herrera <[email protected]>
Cc: Melanie Plageman <[email protected]>
Cc: [email protected]
Subject: Re: pgsql: Add relallfrozen to pg_class
Date: Mon, 3 Mar 2025 11:50:21 -0500
Message-ID: <spdyfkaiav3uzq6qpfiezhqav2x7sqen5i74da2uittljo3z56@br2kogkjyvej> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>

Hi,

On 2025-03-03 17:44:54 +0100, Álvaro Herrera wrote:
> On 2025-Mar-03, Melanie Plageman wrote:
> 
> > Add relallfrozen to pg_class
> > 
> > Add relallfrozen, an estimate of the number of pages marked all-frozen
> > in the visibility map.
> > 
> > pg_class already has relallvisible, an estimate of the number of pages
> > in the relation marked all-visible in the visibility map. This is used
> > primarily for planning.
> > 
> > relallfrozen, together with relallvisible, is useful for estimating the
> > outstanding number of all-visible but not all-frozen pages in the
> > relation for the purposes of scheduling manual VACUUMs and tuning vacuum
> > freeze parameters.
> 
> I'm confused about this.  Why was the new value added to pg_class
> instead of to the pgstat system?  I don't think relallvisible is a good
> precedent, because as you write here, that one is used for planning,
> which has different requirements.  For vacuuming metrics we rely on
> pgstat.

We do so because updating the other stats would cause too much churn - the
number of dead tuples etc changes rather rapidly. That's not the case
here. Having reliable information after a crash (where we loose pgstats) that
95% of the table is actually frozen, rather than not seems good.

Greetings,

Andres Freund

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