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

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.fujii@oss.nttdata.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'

I'm not sure that's a dealbreaker for v17 if we can't come up with anything
else, but IMHO it at least deserves a loud comment and a plan for a better
solution in v18.

-- 
nathan