Re: pg_upgrade and logical replication

Hi,

On Thu, Mar 09, 2023 at 12:05:36PM +0530, Amit Kapila wrote:
> On Wed, Mar 8, 2023 at 12:26 PM Julien Rouhaud <rjuju123(at)gmail(dot)com> wrote:
> >
> > Is there something that can be done for pg16? I was thinking that having a
> > fix for the normal and easy case could be acceptable: only allowing
> > pg_upgrade to optionally, and not by default, preserve the subscription
> > relations IFF all subscriptions only have tables in ready state. Different
> > states should be transient, and it's easy to check as a user beforehand and
> > also easy to check during pg_upgrade, so it seems like an acceptable
> > limitations (which I personally see as a good sanity check, but YMMV). It
> > could be lifted in later releases if wanted anyway.
> >
> > It's unclear to me whether this limited scope would also require to
> > preserve the replication origins, but having looked at the code I don't
> > think it would be much of a problem as the local LSN doesn't have to be
> > preserved.
> >
>
> I think we need to preserve replication origins as they help us to
> determine the WAL location from where to start the streaming after the
> upgrade. If we don't preserve those then from which location will the
> subscriber start streaming?

It would start from the slot's information on the publisher side, but I guess
there's no guarantee that this will be accurate in all cases.

> We don't want to replicate the WAL which
> has already been sent.

Yeah I agree. I added support to also preserve the subscription's replication
origin information, a new --preserve-subscription-state (better naming welcome)
documented option for pg_upgrade to optionally ask for this new mode, and a
similar (but undocumented) option for pg_dump that only works with
--binary-upgrade and added a check in pg_upgrade that all relations are in 'r'
(ready) mode. Patch v2 attached.