A deadlock occurs when two or more users are waiting for data locked by each other. When this happens, these users are stuck (deadly embraced) and cannot continue processing.
Oracle automatically detects deadlocks and resolves them by rolling back one of the statements involved in the deadlock, thus releasing one set of data locked by that statement. The session that is rolled back will observe Oracle error: ORA-00060: deadlock detected while waiting for resource. Oracle will also write out a trace file with detailed information to the database’s UDUMP directory.
Multi-table deadlocks can be avoided by locking tables in same order (in all applications), thus preventing a deadlock condition. For example, session1 lock table: emp then dept; session2: emp then dept. If this is not possible, your application should check for ORA-60 errors and restart the rolled back transactions.