-- UNDOTBS usage per User:
SELECT u.TABLESPACE_NAME          AS TABLESPACE,
       s.USERNAME,
       u.STATUS,
       sum(u.BYTES) / 1024 / 1024 AS SUM_IN_MB,
       count(u.SEGMENT_NAME)      AS SEG_CNTS
FROM DBA_UNDO_EXTENTS u, V$TRANSACTION T, V$SESSION s
WHERE T.ADDR = s.TADDR
GROUP BY u.TABLESPACE_NAME, s.USERNAME, u.STATUS
ORDER BY 1, 2, 3;