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

On Mon, Jul 15, 2024 at 04:03:13PM -0400, Robert Haas wrote:
> On Mon, Jul 15, 2024 at 2:47 PM Nathan Bossart <nathandbossart@gmail.com> wrote:
>> My understanding is that the correctness of this GUC check hook depends on
>> wal_level being a PGC_POSTMASTER GUC.  The check hook would always return
>> true during startup, and there'd be an additional cross-check in
>> PostmasterMain() that would fail startup if necessary.  After that point,
>> we know that wal_level cannot change, so the GUC check hook for
>> summarize_wal can depend on wal_level.  If it fails, my expectation would
>> be that the server would just ignore that change and continue.
> 
> But how do you know that, during startup, the setting for
> summarize_wal is processed after the setting for wal_level?

You don't, but the GUC check hook should always return true when
summarize_wal is processed first.  We'd rely on the PostmasterMain() check
to fail in that case.

-- 
nathan