Date: Wed, 3 Jul 2024 09:29:50 -0500
From: Nathan Bossart <nathandbossart@gmail.com>
To: Fujii Masao <masao.fujii@oss.nttdata.com>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: Add a GUC check hook to ensure summarize_wal cannot be enabled
 when wal_level is minimal
Message-ID: <ZoVgXuYWbzTLXyRY@nathan>
References: <6e30082e-041b-4e31-9633-95a66de76f5d@oss.nttdata.com>
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On Wed, Jul 03, 2024 at 11:08:48PM +0900, Fujii Masao wrote:
> +/*
> + * GUC check_hook for summarize_wal
> + */
> +bool
> +check_summarize_wal(bool *newval, void **extra, GucSource source)
> +{
> +	if (*newval && wal_level == WAL_LEVEL_MINIMAL)
> +	{
> +		GUC_check_errmsg("WAL cannot be summarized when \"wal_level\" is \"minimal\"");
> +		return false;
> +	}
> +	return true;
> +}

IME these sorts of GUC hooks that depend on the value of other GUCs tend to
be quite fragile.  This one might be okay because wal_level defaults to
'replica' and because there is an additional check in postmaster.c, but
that at least deserves a comment.

This sort of thing comes up enough that perhaps we should add a
better-supported way to deal with GUCs that depend on each other...

-- 
nathan