21:07:10 SQL> select view_name from dba_views where view_name like '%UNDO%';

VIEW_NAME
------------------------------
V_$UNDOSTAT
GV_$UNDOSTAT
DBA_UNDO_EXTENTS
DBA_HIST_UNDOSTAT

21:07:38 SQL> 

SQL> select sum(bytes/1024/1024) from dba_undo_extents;

SUM(BYTES/1024/1024)
--------------------
              354.25

SQL> select sum(bytes/1024/1024) from dba_free_space where tablespace_name='UNDO';

SUM(BYTES/1024/1024)
--------------------
            145.6875

SQL> select sum(bytes/1024/1024), autoextensible from dba_data_files where tablespace_name='UNDO' group by autoextensible;

SUM(BYTES/1024/1024) AUT
-------------------- ---
                 500 NO
SQL> select 145.6875 + 354.25 from dual;

145.6875+354.25
---------------
       499.9375