Re: pg_restore deadlocks with itself

On Tue, Aug 30, 2022 at 11:06:34AM +0200, Alvaro Herrera wrote:
> On 2022-Aug-25, hubert depesz lubaczewski wrote:
> 
> > Specifically, command to run it is:
> > sudo -u postgres pg_restore -j 64 -d database -L /tmp/schema-post-data.nopkey.list /tmp/schema-post-data.dump
> 
> > 2022-08-24 20:01:04.466 UTC,"postgres","database",3343477,"[local]",630624ad.330475,42,"ALTER TABLE
waiting",2022-08-2413:16:29 UTC,21/1932,0,LOG,00000,"process 3343477 detected deadlock while waiting for
ShareRowExclusiveLockon relation 742617610 of database 16641 after 1000.647 ms","Process holding the lock: 3587718.
Waitqueue: .",,,,,"ALTER TABLE ONLY some_schema.table_a_o
 
> >     ADD CONSTRAINT table_a_o_q_id_fk FOREIGN KEY (q_id) REFERENCES some_schema.table_q(id);
> > ",,,"pg_restore","client backend",,3355460102417501954
> > 
> > 
> > 2022-08-24 20:01:50.291 UTC,"postgres","database",3343477,"[local]",630624ad.330475,46,"ALTER TABLE
waiting",2022-08-2413:16:29 UTC,21/1933,0,LOG,00000,"process 3343477 detected deadlock while waiting for
ShareRowExclusiveLockon relation 742617610 of database 16641 after 1000.030 ms","Process holding the lock: 3587718.
Waitqueue: .",,,,,"ALTER TABLE ONLY some_schema.table_a
 
> >     ADD CONSTRAINT fk_rails_46718e626a FOREIGN KEY (migrate_from_id) REFERENCES some_schema.table_q(id);
> > ",,,"pg_restore","client backend",,-2548896815899838768
> > 
> > Now, I know I can fix the situation by adding missing fkeys myself, but
> > I don't think pg_restore should be putting itself in deadlock.
> 
> You're right, it shouldn't.  Parallel restore shouldn't run DDL commands
> in parallel that would deadlock, but I suppose there must be holes in
> that.
> 
> What was process 3587718 doing at the time?  Some DDL in
> some_schema.table_q, I suspect, right?  Are any of these tables
> partitioned?

So, more info from logs:

2022-08-24 20:01:04.466 UTC,"postgres","database",3343477,"[local]",630624ad.330475,43,"ALTER TABLE",2022-08-24
13:16:29UTC,21/1932,0,ERROR,40P01,"deadlock detected","Process 3343477 waits for ShareRowExclusiveLock on relation
742617610of database 16641; blocked by process 3587718.
 
Process 3587718 waits for RowExclusiveLock on relation 742615338 of database 16641; blocked by process 3343477.
Process 3343477: ALTER TABLE ONLY some_schema.table_ao
    ADD CONSTRAINT table_ao_q_id_fk FOREIGN KEY (q_id) REFERENCES some_schema.table_q(id);



Process 3587718: <command string not enabled>","See server log for query details.",,,,"ALTER TABLE ONLY
some_schema.table_ao
    ADD CONSTRAINT table_ao_q_id_fk FOREIGN KEY (q_id) REFERENCES some_schema.table_q(id);


",,,"pg_restore","client backend",,3355460102417501954

Which is weird, as it seems that they both were trying to do the same fkey?!

Neither table_ao nor table_q are partitioned, and used restore-list contained this fkey only once.

Best regards,

depesz