Horton

Re: vacuum_truncate configuration parameter and isset_offset

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Nikolay Shaplov <dhyan@nataraj.su>
Cc: Robert Haas <robertmhaas@gmail.com>, Álvaro Herrera <alvherre@alvh.no-ip.org>, "David G. Johnston" <david.g.johnston@gmail.com>, pgsql-hackers@lists.postgresql.org, Robert Treat <rob@xzilla.net>, Fujii Masao <masao.fujii@oss.nttdata.com>, Albe <laurenz.albe@cybertec.at>, Gurjeet Singh <gurjeet@singh.im>, Will Storey <will@summercat.com>, Tom Lane <tgl@sss.pgh.pa.us>
Date: 2025-03-24T20:04:39Z
Lists: pgsql-hackers 
On Mon, Mar 24, 2025 at 10:35:41PM +0300, Nikolay Shaplov wrote:
> We can have isset_offset, but then we have redesign all options with
> custom unset behavior to use it, instead of unreachable default value.
> This will make it consistent then.

I don't see any reason why we are compelled to redesign all such options,
but in any case, I would think that would be preferable to magic special
values.  For example,

	/* -1 is used to disable max threshold */
	vac_max_thresh = (relopts && relopts->vacuum_max_threshold >= -1)
		? relopts->vacuum_max_threshold
		: autovacuum_vac_max_thresh;

would become something like

	if (relopts && relopts->vacuum_max_threshold_set)
		vac_max_thresh = relopts->vacuum_max_threshold;
	else
		vac_max_thresh = autovacuum_vac_max_thresh;

The former requires you to know that the reloption defaults to -2 if not
set, which does not seem particularly obvious to me.  At least, I did not
find this out-of-range reloption default technique obvious when I was
working on autovacuum_vacuum_max_threshold.

But again, I don't see any strong reason why we must change all such
reloptions.

-- 
nathan