关于估算undo表空间的大小的命令：SUM(undoblks) / SUM( ((end_time - begin_time) *

在资料上看到估算undo表空间的大小的命令： SELECT (RD * (UPS * OVERHEAD) + OVERHEAD) AS "Bytes“ FROM (SELECT value AS RD FROM v$parameter WHERE name = 'undo_retention'), (SELECT (SUM(undoblks) / SUM( ((end_time - begin_time) * 86400))) AS UPS FROM v$undostat), (SELECT value AS Overhead FROM v$parameter WHERE name = 'db_block_size'); 不明白这里为什么要乘以86400，即一天的秒数 (SUM(undoblks) / SUM( ((end_time - begin_time) * 86400)))