Re: Add a GUC check hook to ensure summarize_wal cannot be enabled when wal_level is minimal

On 2024/07/10 23:18, Nathan Bossart wrote:
> On Wed, Jul 10, 2024 at 10:10:30AM -0400, Robert Haas wrote:
>> On Wed, Jul 10, 2024 at 1:56 AM Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> wrote:
>>> I'm sure this patch is necessary as a safeguard for WAL summarization.
>>> OTOH, I also think we should apply the patch I proposed earlier
>>> in this thread, which prevents summarize_wal from being enabled
>>> when wal_level is set to minimal. This way, if there's
>>> a misconfiguration, users will see an error message and
>>> can quickly identify and fix the issue. Thought?
>>
>> I interpreted these emails as meaning that we should not proceed with
>> that approach:
>>
>> https://www.postgresql.org/message-id/CAGECzQR2r-rHFLQr5AonFehVP8DiFH+==R2yqdBvunYnwxsXNA@mail.gmail.com
>> http://postgr.es/m/3253790.1720019802@sss.pgh.pa.us
>
> Yeah. I initially thought this patch might be okay, at least as a stopgap,
> but Jelte pointed out a case where it doesn't work, namely when you have
> something like the following in the config file:
>
> wal_level = 'minimal'
> summarize_wal = 'true'
> wal_level = 'logical'

Unless I'm mistaken, the patch works fine in this case. If the check_hook
triggered every time a parameter appears in the configuration file,
it would mistakenly detect wal_level=minimal and summarize_wal=on together
and raise an error. However, this isn't the case. The check_hook is
designed to trigger after duplicate parameters are deduplicated.
Am I missing something?

Regards,

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