On Tue, Jul 26, 2022 at 2:30 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Fri, Jul 22, 2022 at 8:27 AM wangw.fnst@fujitsu.com
> <wangw.fnst@fujitsu.com> wrote:
> >
> > On Tues, Jul 19, 2022 at 10:29 AM I wrote:
> > > Attach the news patches.
> >
> > Not able to apply patches cleanly because the change in HEAD (366283961a).
> > Therefore, I rebased the patch based on the changes in HEAD.
> >
> > Attach the new patches.
>
> + /* Check the foreign keys. */
> + fkeys = RelationGetFKeyList(entry->localrel);
> + if (fkeys)
> + entry->parallel_apply = PARALLEL_APPLY_UNSAFE;
>
> So if there is a foreign key on any of the tables which are parts of a
> subscription then we do not allow changes for that subscription to be
> applied in parallel?
>
I think the above check will just prevent the parallelism for a xact
operating on the corresponding relation not the relations of the
entire subscription. Your statement sounds like you are saying that it
will prevent parallelism for all the other tables in the subscription
which has a table with FK.
> I think this is a big limitation because having
> foreign key on the table is very normal right? I agree that if we
> allow them then there could be failure due to out of order apply
> right?
>
What kind of failure do you have in mind and how it can occur? The one
way it can fail is if the publisher doesn't have a corresponding
foreign key on the table because then the publisher could have allowed
an insert into a table (insert into FK table without having the
corresponding key in PK table) which may not be allowed on the
subscriber. However, I don't see any check that could prevent this
because for this we need to compare the FK list for a table from the
publisher with the corresponding one on the subscriber. I am not
really sure if due to the risk of such conflicts we should block
parallelism of transactions operating on tables with FK because those
conflicts can occur even without parallelism, it is just a matter of
timing. But, I could be missing something due to which the above check
can be useful?
--
With Regards,
Amit Kapila.
