如何计算必要的回滚段大小

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 v$datafile a,
              v$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,
       v$parameter e,
       v$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'
  ;
 
  SELECT (UR * (UPS * DBS)) + (DBS * 24) AS "Bytes"
    FROM (SELECT value AS UR
            FROM v$parameter
           WHERE name = 'undo_retention'),
           (SELECT (SUM(undoblks)/SUM
                    (((end_time-begin_time)*86400))) AS UPS
           FROM v$undostat),
            (SELECT value AS DBS
           FROM v$parameter
           WHERE name = 'db_block_size');

(UR) UNDO_RETENTION in seconds
(UPS) Number of undo data blocks generated per second
(DBS) Overhead varies based on extent and file size (db_block_size)

UndoSpace = [UR * (UPS * DBS)] + (DBS * 24)

======================
The UNDO tablespace still must be sized appropriately.
The following calculation can be used to determine how much space a given undo segment will consume
given a set value of UNDO_RETENTION.
Undo Segment Space Required = (undo_retention_time * undo_blocks_per_seconds)
As an example, an UNDO_RETENTION of 5 minutes (default) with 50 undo blocks/second (8k blocksize) will generate:
Undo Segment Space Required = (300 seconds * 50 blocks/ seconds * 8K/block) = 120 M