Dead Locks

I am receiveing dead locks like this:

Jul 22, 2014 11:00:29 PM br.com.fastway.fastdialer.FastDialerDB query
SEVERE: SELECT * FROM ONGOING WHERE STATUS='FILA' ORDER BY TRUNK_ID,
PRIORIDADE_TRONCO, ID;
org.postgresql.util.PSQLException: ERROR: deadlock detected
Detail: Process 508 waits for AccessShareLock on relation 2662 of
database 16384; blocked by process 8504.
Process 8504 waits for AccessExclusiveLock on relation 2663 of database
16384; blocked by process 508.

What seems odd is that relations 2662 and 2663 are not user tables but
postgres objects.

select oid, relname from pg_class where oid in (2662,2663)

2662;"pg_class_oid_index"
2663;"pg_class_relname_nsp_index"

Futhermore, this system don't use transactions (I mean it is autocommit)
and we don't use triggers, rules or even foreign keys contraints.

Thanks for any help.

Valter