Re: Support logical replication of DDLs

On Wed, Jun 29, 2022 at 3:17 PM houzj(dot)fnst(at)fujitsu(dot)com
<houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>
> On Tuesday, June 28, 2022 11:27 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> > On Sun, Jun 26, 2022 at 11:47 PM Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
> > wrote:
> > >
> > > However, that would still replicate a command that involves a
> > > temporary table, which perhaps should not be considered fit for
> > > replication. So another school of thought is that if the
> > > %{persistence} is set to TEMPORARY, then it would be better to skip
> > > replicating the command altogether.
> > >
> >
> > +1. I think it doesn't make sense to replicate temporary tables.
> > Similarly, we don't need to replicate the unlogged tables.
>
> I agree that we don’t need to replicate temporary tables.
>
> For unlogged table, one thing I noticed is that we always replicate the
> DDL action on unlogged table in streaming replication. So, to be
> consistent, maybe we need to generate WAL for DDL on unlogged table as
> well ?
>

We don't seem to WAL log the main fork, so that shouldn't be created
in physical replication whereas in your case it will create the main
fork unless you are doing some special handling for subscribers/apply
worker. We are also not allowed to read the unlogged tables on standby
whereas after logical replication users will be allowed to operate on
them. I think because we need to insert catalog entries for 'create
unlogged table' which can't be selectively logged, it gets replicated
to a physical stand by but I don't see why we need to behave similarly
for logical replication. Can you think of some reason why we need to
be consistent here or in other words why we should replicate DDL for
unlogged tables in logical replication? I am not against it but can't
see the reason for doing it based on the theory that when we are not
going to replicate the data of such tables why should we replicate its
schema.

--
With Regards,
Amit Kapila.