On Wed, May 19, 2021 at 7:59 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Tue, May 18, 2021 at 5:29 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Tue, May 18, 2021 at 1:29 PM osumi.takamichi@fujitsu.com
> > <osumi.takamichi@fujitsu.com> wrote:
> > >
> > > On Monday, May 17, 2021 6:45 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > >
> > > > We allow taking locks on system catalogs, so why prohibit
> > > > user_catalog_tables? However, I agree that if we want plugins to acquire the
> > > > lock on user_catalog_tables then we should either prohibit decoding of such
> > > > relations or do something else to avoid deadlock hazards.
> > > OK.
> > >
> > > Although we have not concluded the range of logical decoding of user_catalog_table
> > > (like we should exclude TRUNCATE command only or all operations on that type of table),
> > > I'm worried that disallowing the logical decoding of user_catalog_table produces
> > > the deadlock still. It's because disabling it by itself does not affect the
> > > lock taken by TRUNCATE command. What I have in mind is an example below.
> > >
> > > (1) plugin (e.g. pgoutput) is designed to take a lock on user_catalog_table.
> > > (2) logical replication is set up in synchronous mode.
> > > (3) TRUNCATE command takes an access exclusive lock on the user_catalog_table.
> > > (4) This time, we don't do anything for the TRUNCATE decoding.
> > > (5) the plugin tries to take a lock on the truncated table
> > > but, it can't due to the lock by TRUNCATE command.
> > >
> >
> > If you skip decoding of truncate then we won't invoke plugin API so
> > step 5 will be skipped.
> >
>
> I think you were right here even if skip step-4, the plugin might take
> a lock on user_catalog_table for something else. I am not sure but I
> think we should prohibit truncate on user_catalog_tables as we
> prohibit truncate on system catalog tables (see below [1]) if we want
> plugin to lock them, otherwise, as you said it might lead to deadlock.
> For the matter, I think we should once check all other operations
> where we can take an exclusive lock on [user]_catalog_table, say
> Cluster command, and compare the behavior of same on system catalog
> tables.
>
> [1]
> postgres=# truncate pg_class;
> ERROR: permission denied: "pg_class" is a system catalog
> postgres=# cluster pg_class;
> ERROR: there is no previously clustered index for table "pg_class"
>
Please ignore the cluster command as we need to use 'using index' with
that command to make it successful. I just want to show the truncate
command behavior for which you have asked the question.
--
With Regards,
Amit Kapila.