In Recovery Catalog Database:
If the target database already deleted/dropped/decommissioned or not available
Then in 9i/10g
SQL> alter session set current_schema=RCAT10G_OTH;
Session altered.
SQL> execute dbms_rcvcat.unregisterdatabase(1,129982387);
PL/SQL procedure successfully completed.
If Target database is available and UP, Then in 10g
rman>connect target id/pwd@dbname
rman>connect catalog id/pwd@catalogdb
RMAN> UNREGISTER DATABASE ;
