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

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