On Wed, Jun 17, 2020 at 12:27 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Which of the things I mentioned don't require writing WAL?
Writing hint bits and marking index tuples as killed do not write WAL
unless checksums are enabled.
> You're right that these are the same things that we already forbid on a
> standby, for the same reason, so maybe it won't be as hard to identify
> them as I feared. I wonder whether we should envision this as "demote
> primary to standby" rather than an independent feature.
See my comments on the nearby pg_demote thread. I think we want both.
> >> I also think that putting such a thing into ALTER SYSTEM has got big
> >> logical problems.
>
> > ... no right-thinking person would ever propose to
> > change a feature that renders the system read-only in such a way that
> > it was impossible to deactivate it. That would be nuts.
>
> My point was that putting this in ALTER SYSTEM paints us into a corner
> as to what we can do with ALTER SYSTEM in the future: we won't ever be
> able to make that do anything that would require writing WAL. And I
> don't entirely believe your argument that that will never be something
> we'd want to do.
I think that depends a lot on how you view ALTER SYSTEM. I believe it
would be reasonable to view ALTER SYSTEM as a catch-all for commands
that make system-wide state changes, even if those changes are not all
of the same kind as each other; some might be machine-local, and
others cluster-wide; some WAL-logged, and others not. I don't think
it's smart to view ALTER SYSTEM through a lens that boxes it into only
editing postgresql.auto.conf; if that were so, we ought to have called
it ALTER CONFIGURATION FILE or something rather than ALTER SYSTEM. For
that reason, I do not see the choice of syntax as painting us into a
corner.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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