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From: Fujii Masao <masao.fujii@oss.nttdata.com>
To: Nathan Bossart <nathandbossart@gmail.com>
To: Robert Haas <robertmhaas@gmail.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
Date: Wed, 17 Jul 2024 02:16:27 +0900
Message-ID: <22585eb0-4a10-4d99-9914-bdee011e8778@oss.nttdata.com> (raw)
In-Reply-To: <ZpagGUyMkwg-LeKC@nathan>
References: <CA+TgmoansG0B8ayQGw9Ozee6Jawz=HFUACpO_4gp-+SFJBxiYw@mail.gmail.com>
	<CA+TgmoZ53ADUm2UAGEspMuv-Db5SGtrOXUx=ps6LcGexsf5-fA@mail.gmail.com>
	<e2549085-5246-4edc-bc62-ae19df48acc5@oss.nttdata.com>
	<CA+TgmoYFOWbxk=FP+ZnxwVaKQEce=+0Xvzh0DUC9=YLNHiQW1w@mail.gmail.com>
	<8a668c51-c6af-4744-a978-c6a2ce3b1d83@oss.nttdata.com>
	<CA+TgmoYUhRa8j8_Jdrb9Ankz=c8NRLe_x1nf4L+Vc2V+DiTSWw@mail.gmail.com>
	<ZpVushkGx0_nYIip@nathan>
	<CA+TgmoaKqLrkjEg1j6NEHcMUj-Qk6yUK=qfFzy+G-w6OVVv+0Q@mail.gmail.com>
	<ZpWCJxL3tQm3lkLI@nathan>
	<CA+TgmobLzdVQCjy9abec-UFrN1SmwHGv9WcaOD_XixmJN5e+=A@mail.gmail.com>
	<ZpagGUyMkwg-LeKC@nathan>



On 2024/07/17 1:30, Nathan Bossart wrote:
> On Tue, Jul 16, 2024 at 12:23:19PM -0400, Robert Haas wrote:
>> TBH, I don't want to do that. I think it's too fragile. It's the sort
>> of thing that just barely works given the exact behavior of these
>> particular GUCs, but it relies on a bunch of subtle assumptions which
>> won't be evident to future readers of the code. People will very
>> possibly copy this barely-working code into other contexts where it
>> doesn't work at all, or they'll think the code implementing this is
>> buggy even if it isn't.
> 
> Agreed.  If there was really no other option, it would at the very least
> need a humongous comment that explained why it worked in this specific case
> and is unlikely to work in others.  But it sounds like we have another
> choice...

I don't have another solution that can be pushed into v17. I understand
the risks raised so far, so I'm okay with just pushing the "fast_forward" patch.
It might be helpful to add a note to the summarize_wal documentation,
for example, "summarize_wal can be enabled after startup with wal_level = minimal,
but WAL generated at this level won't be summarized."?

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION





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