SQL> select sum(d.bytes/1024/1024) "undo_mb" from v$datafile d, v$tablespace t, dba_tablespaces s where s.contents = 'UNDO' and s.status = 'ONLINE' and t.name = s.tablespace_name and d.ts# = t.ts#; undo_mb
----------
102400
SQL> show parameter db_block_size NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ db_block_size integer 8192
--以秒来计算undo_retention参数的值:
SQL> select 102400*1024*1024/(272.980265*8192) from dual;