SELECT d.undo_size / (1024 * 1024) "ACTUAL UNDO SIZE [MByte]",
SUBSTR(e.value, 1, 25) "UNDO RETENTION [Sec]",
(TO_NUMBER(e.value) * TO_NUMBER(f.value) * g.undo_block_per_sec) /
(1024 * 1024) "NEEDED UNDO SIZE [MByte]"
FROM (SELECT SUM(a.bytes) undo_size
FROM gv$datafile a, gv$tablespace b, dba_tablespaces c
WHERE c.contents = 'UNDO'
AND c.status = 'ONLINE'
AND b.name = c.tablespace_name
AND a.ts# = b.ts#) d,
gv$parameter e,
gv$parameter f,
(SELECT MAX(undoblks / ((end_time - begin_time) * 3600 * 24)) undo_block_per_sec
FROM v$undostat) g
WHERE e.name = 'undo_retention'
AND f.name = 'db_block_size';