UNDO tablespace reaching maximum size - WHO is using it?

Question

My UNDO tablespace has been pretty static for a while but this morning it was at 93% and within 20 minutes I saw it at 98%. I added a datafile to buy some time, but is there any way I can identify specific sessions which may be filling it rapidly ?
Thanks

Answer 1

Thank you Pierre! I must have missed that particular piece. Mind you, it was a rather long thread!
select sum(used_ublk) from v$transaction
This now works..
SELECT   SUBSTR (terminal, 1, 10) terminal, SUBSTR (osuser, 1, 15) osuser,
         SUBSTR (SID, 1, 3) SID, SUBSTR (username, 1, 18) username,
         SUBSTR (program, 1, 30) program,
         TO_CHAR (logon_time, 'DD/MM/YYYY HH24:MI:SS') logon_time,
         SUBSTR (command, 1, 20) command, SUM (used_ublk) undo_used
    FROM (SELECT   s.terminal, s.osuser, s.SID, s.username, s.program,
                   s.logon_time,
                   DECODE (s.command,
                           0, 'No Command',
                           1, 'Create Table',
                           2, 'Insert',
                           3, 'Select',
                           6, 'Update',
                           7, 'Delete',
                           9, 'Create Index',
                           15, 'Alter Table',
                           21, 'Create View',
                           23, 'Validate Index',
                           35, 'Alter Database',
                           39, 'Create Tablespace',
                           41, 'Drop Tablespace',
                           40, 'Alter Tablespace',
                           53, 'Drop User',
                           62, 'Analyze Table',
                           63, 'Analyze Index',
                           s.command || ': Other'
                          ) command,
                   used_ublk
              FROM v$session s,
                   v$process p,
                   v$transaction t,
                   v$rollstat r,
                   v$rollname n
             WHERE s.paddr = p.addr
               AND s.taddr = t.addr(+)
               AND t.xidusn = r.usn(+)
               AND r.usn = n.usn(+)
          ORDER BY 1)
GROUP BY terminal, osuser, SID, logon_time, username, program, command
  HAVING SUM (used_ublk) > 0
ORDER BY SUM (used_ublk) DESC