On Wed, May 19, 2021 at 7:59 AM Amit Kapila <amit.kapil...@gmail.com> wrote: > > On Tue, May 18, 2021 at 5:29 PM Amit Kapila <amit.kapil...@gmail.com> wrote: > > > > On Tue, May 18, 2021 at 1:29 PM osumi.takami...@fujitsu.com > > <osumi.takami...@fujitsu.com> wrote: > > > > > > On Monday, May 17, 2021 6:45 PM Amit Kapila <amit.kapil...@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.
