Undo Retention

SQL> alter tablespace undotbs2 retention guarantee;

SQL> alter tablespace undotbs2 retention noguarantee;

SQL> alter tablespace undotbs2 offline;

                ORA-30042 : cannot offline undo tablespace

SQL> alter tablespace undotbs1 offline;

SQL> alter tablespace undotbs1 online;

SQL> alter system set undo_tablespace=undotbs1;