确定最优的撤销保留时间 undo_rentention

SQL> show parameter undo
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
undo_management                      string      AUTO
undo_retention                       integer     900
undo_tablespace                      string      UNDOTBS1
SQL> alter system set undo_retention=1800 scope=both
 
SQL> show parameter undo
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
undo_management                      string      AUTO
undo_retention                       integer     1800
undo_tablespace                      string      UNDOTBS1

SQL> select sum(d.bytes) "undo" 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
  ----------
  31457280

SQL> select max(undoblks/((end_time-begin_time)*3600*24)) "UNDO_BLOCK_PER_SEC" from v$undostat;


UNDO_BLOCK_PER_SEC
------------------
        2.90217391