Re: Option to dump foreign data in pg_dump

On Tue, Jan 14, 2020 at 5:22 PM Luis Carril <luis(dot)carril(at)swarm64(dot)com> wrote:

> Can you have a look at dump with parallel option. Parallel option will
> take a lock on table while invoking lockTableForWorker. May be this is
> not required for foreign tables.
> Thoughts?
>
> I tried with -j and found no issue. I guess that the foreign table needs
> locking anyway to prevent anyone to modify it while is being dumped.
>
>
I'm able to get the problem with the following steps:
Bring up a postgres setup with servers running in 5432 & 5433 port.

Execute the following commands in Server1 configured on 5432 port:

- CREATE EXTENSION postgres_fdw;

- CREATE SERVER foreign_server FOREIGN DATA WRAPPER postgres_fdw OPTIONS
(host '127.0.0.1', port '5433', dbname 'postgres');

- create user user1 password '123';

- alter user user1 with superuser;

- CREATE USER MAPPING FOR user1 SERVER foreign_server OPTIONS (user
'user1', password '123');

Execute the following commands in Server2 configured on 5433 port:

- create user user1 password '123';

- alter user user1 with superuser;

Execute the following commands in Server2 configured on 5433 port as user1
user:

- create schema test;

- create table test.test1(id int);

- insert into test.test1 values(10);

Execute the following commands in Server1 configured on 5432 port as user1
user:

- CREATE FOREIGN TABLE foreign_table1 (id integer NOT NULL) SERVER
foreign_server OPTIONS (schema_name 'test', table_name 'test1');

Without parallel option, the operation is successful:

- ./pg_dump -d postgres -f dumpdir -U user1 -F d --include-foreign-data
foreign_server

With parallel option it fails:

- ./pg_dump -d postgres -f dumpdir1 -U user1 -F d -j 5
--include-foreign-data foreign_server

pg_dump: error: could not obtain lock on relation "public.foreign_table1"
This usually means that someone requested an ACCESS EXCLUSIVE lock on the
table after the pg_dump parent process had gotten the initial ACCESS SHARE
lock on the table.
pg_dump: error: a worker process died unexpectedly

There may be simpler steps than this to reproduce the issue, i have not try
to optimize it.

Regards,
Vignesh
EnterpriseDB: http://www.enterprisedb.com