oracle undo usage per session
-- 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;