Oracle 11g new feature: PRIMARY_DB_UNIQUE_NAME column in V$DATABASE

You can now find the DB_UNIQUE_NAME of the primary database from the standby database by querying the new PRIMARY_DB_UNIQUE_NAME column in the V$DATABASE view.

Also, Oracle Data Guard release 11g ensures each database's DB_UNIQUE_NAME is different.

It means, after upgrading to 11g, any databases with the same DB_UNIQUE_NAME will not be able to communicate with each other.

SQL> desc v$database
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------

 DBID                                               NUMBER
 NAME                                               VARCHAR2(9)
 CREATED                                            DATE
 RESETLOGS_CHANGE#                                  NUMBER
 RESETLOGS_TIME                                     DATE
 PRIOR_RESETLOGS_CHANGE#                            NUMBER
 PRIOR_RESETLOGS_TIME                               DATE
 LOG_MODE                                           VARCHAR2(12)
 CHECKPOINT_CHANGE#                                 NUMBER
 ARCHIVE_CHANGE#                                    NUMBER
 CONTROLFILE_TYPE                                   VARCHAR2(7)
 CONTROLFILE_CREATED                                DATE
 CONTROLFILE_SEQUENCE#                              NUMBER
 CONTROLFILE_CHANGE#                                NUMBER
 CONTROLFILE_TIME                                   DATE
 OPEN_RESETLOGS                                     VARCHAR2(11)
 VERSION_TIME                                       DATE
 OPEN_MODE                                          VARCHAR2(20)
 PROTECTION_MODE                                    VARCHAR2(20)
 PROTECTION_LEVEL                                   VARCHAR2(20)
 REMOTE_ARCHIVE                                     VARCHAR2(8)
 ACTIVATION#                                        NUMBER
 SWITCHOVER#                                        NUMBER
 DATABASE_ROLE                                      VARCHAR2(16)
 ARCHIVELOG_CHANGE#                                 NUMBER
 ARCHIVELOG_COMPRESSION                             VARCHAR2(8)
 SWITCHOVER_STATUS                                  VARCHAR2(20)
 DATAGUARD_BROKER                                   VARCHAR2(8)
 GUARD_STATUS                                       VARCHAR2(7)
 SUPPLEMENTAL_LOG_DATA_MIN                          VARCHAR2(8)
 SUPPLEMENTAL_LOG_DATA_PK                           VARCHAR2(3)
 SUPPLEMENTAL_LOG_DATA_UI                           VARCHAR2(3)
 FORCE_LOGGING                                      VARCHAR2(3)
 PLATFORM_ID                                        NUMBER
 PLATFORM_NAME                                      VARCHAR2(101)
 RECOVERY_TARGET_INCARNATION#                       NUMBER
 LAST_OPEN_INCARNATION#                             NUMBER
 CURRENT_SCN                                        NUMBER
 FLASHBACK_ON                                       VARCHAR2(18)
 SUPPLEMENTAL_LOG_DATA_FK                           VARCHAR2(3)
 SUPPLEMENTAL_LOG_DATA_ALL                          VARCHAR2(3)
 DB_UNIQUE_NAME                                     VARCHAR2(30)
 STANDBY_BECAME_PRIMARY_SCN                         NUMBER
 FS_FAILOVER_STATUS                                 VARCHAR2(22)
 FS_FAILOVER_CURRENT_TARGET                         VARCHAR2(30)
 FS_FAILOVER_THRESHOLD                              NUMBER
 FS_FAILOVER_OBSERVER_PRESENT                       VARCHAR2(7)
 FS_FAILOVER_OBSERVER_HOST                          VARCHAR2(512)
 CONTROLFILE_CONVERTED                              VARCHAR2(3)
 PRIMARY_DB_UNIQUE_NAME                             VARCHAR2(30)
 SUPPLEMENTAL_LOG_DATA_PL                           VARCHAR2(3)
 MIN_REQUIRED_CAPTURE_CHANGE#                       NUMBER

SQL>

Quote from documentation(http://download.oracle.com/docs/cd/B28359_01/server.111/b28320/dynviews_1086.htm#I1030047):

PRIMARY_DB_UNIQUE_NAME VARCHAR2(30): For any Standby database (Physical, Logical, or Reporting), this column will contain the DB_UNIQUE_NAME of the Primary database that this Standby last received current redo from.

If this standby has not received any current redo since last being started, then this column will be null.

For a Primary database that had previously been a Standby, this column will contain the DB_UNIQUE_NAME of the last Primary that this database received current redo from while acting as a Standby.

For a Primary database that has never been a Standby, this column will be null.