oracle 查询谁在用undo

SELECT TO_CHAR(s.sid)||','||TO_CHAR(s.serial#) sid_serial,
NVL(s.username, 'None') orauser,
s.program, r.name undoseg,
t.used_ublk * TO_NUMBER(x.value)/1024/1024||'M' "Undo"
FROM sys.v_$rollname r,
sys.v_$session s,
sys.v_$transaction t,
sys.v_$parameter x
WHERE s.taddr = t.addr AND r.usn = t.xidusn(+)
AND x.name = 'db_block_size'