Re: Logical Replication - behavior of ALTER PUBLICATION .. DROP TABLE and ALTER SUBSCRIPTION .. REFRESH PUBLICATION - Mailing list pgsql-hackers

On Fri, Jan 15, 2021 at 11:33 AM Hou, Zhijie <houzj.fnst@cn.fujitsu.com> wrote:
>
> > On Thu, Jan 14, 2021 at 5:36 PM Li Japin <japinli@hotmail.com> wrote
> > > Do we really need to access PUBLICATIONRELMAP in this patch? What if
> > > we just set it to false in the else condition of (if (publish &&
> > > (relkind != RELKIND_PARTITIONED_TABLE || pub->pubviaroot)))
> > >
> > > Thank for you review. I agree with you, it doesn’t need to access
> > > PUBLICATIONRELMAP, since We already get the publication oid in
> > > GetRelationPublications(relid) [1], which also access
> > > PUBLICATIONRELMAP.  If the current pub->oid does not in pubids, the
> > publish is false, so we do not need publish the table.
> >
> > +1. This is enough.
> >
> > > I have another question, the data->publications is a list, when did it
> > has more then one items?
> >
> > IIUC, when the single table is associated with multiple publications, then
> > data->publications will have multiple entries. Though I have not tried,
> > we can try having two or three publications for the same table and verify
> > that.
> >
> > > 0001 - change as you suggest.
> > > 0002 - does not change.
>
>
> Hi
>
> In 0001 patch
>
> The comments says " Relation is not associated with the publication anymore "
> That comment was accurate when check is_relation_part_of_publication() in the last patch.
>
> But when put the code in the else branch, not only the case in the comments(Relation is dropped),
> Some other case such as "partitioned tables" will hit else branch too.
>
> Do you think we need fix the comment a little to make it accurate?

Thanks, that comment needs a rework, in fact the else condition
also(because we may fall into else branch even when publish is true
and (relkind != RELKIND_PARTITIONED_TABLE || pub->pubviaroot) is
false, but our intention(to fix the bug reported here) is to have the
else condition only when publish is false. And also instead of just
relying on the publish variable which can get set even when the
relation is not in the publication but ancestor_published is true, we
can just have something like below to fix the bug reported here:

            if (publish &&
                (relkind != RELKIND_PARTITIONED_TABLE || pub->pubviaroot))
            {
                entry->pubactions.pubinsert |= pub->pubactions.pubinsert;
                entry->pubactions.pubupdate |= pub->pubactions.pubupdate;
                entry->pubactions.pubdelete |= pub->pubactions.pubdelete;
                entry->pubactions.pubtruncate |= pub->pubactions.pubtruncate;
            }
            else if (!list_member_oid(pubids, pub->oid))
            {
                /*
                 * Relation is not associated with the publication anymore i.e
                 * it would have been dropped from the publication. So we don't
                 * need to publish the changes for it.
                 */
                entry->pubactions.pubinsert = false;
                entry->pubactions.pubupdate = false;
                entry->pubactions.pubdelete = false;
                entry->pubactions.pubtruncate = false;
            }

So this way, the fix only focuses on what we have reported here and it
doesn't cause any regressions may be, and the existing comment becomes
appropriate.

Thoughts?

> And it seems we can add a "continue;" in else branch.
> Of course this it not necessary.

As you said, it's not necessary because the following if condition
will fail anyways. Having said that, if we add continue, then any
future code that gets added after the following if condition will
never get executed. Though the reasoning may sound silly, IMO let's
not add the continue in the else branch.

            if (entry->pubactions.pubinsert && entry->pubactions.pubupdate &&
                entry->pubactions.pubdelete && entry->pubactions.pubtruncate)
                break;

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com