what is SMON_SCN_TIME ?

SMON_SCN_TIME是Oracle数据库的系统表，用以进行辅助恢复等功能。
在Oracle9iR2中,SMON_SCN_TIME每5分钟被更新一次。

[oracle@jumper oracle]$ sqlplus "/ as sysdba"
SQL*Plus: Release 9.2.0.4.0 - Production on Mon Dec 19 18:19:27 2005
Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.

Connected to:
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production
SQL> alter session set nls_date_format='yyyy-mm-dd hh24:mi:ss';
Session altered.
SQL> select * from smon_scn_time where rownum <10;
    THREAD    TIME_MP TIME_DP                SCN_WRP    SCN_BAS
---------- ---------- ------------------- ---------- ----------
         1 1134916635 2005-12-18 22:37:15         25      65931
         1 1134916648 2005-12-18 22:37:28         25      65979
         1 1134917029 2005-12-18 22:43:49         25      66224
         1 1134917336 2005-12-18 22:48:57         25      66325
         1 1134917643 2005-12-18 22:54:04         25      66426
         1 1134917950 2005-12-18 22:59:11         25      66527
         1 1134918257 2005-12-18 23:04:18         25      66628
         1 1134918564 2005-12-18 23:09:25         25      66733
         1 1134918871 2005-12-18 23:14:33         25      66836
9 rows selected.

SMON_SCN_TIME记录5天的数据，也就是1440 ((5d x 24h x 12m = 1440) 条记录。
所以在Oracle9iR2中，表属性修改时间和flashback时间差至少应为5分钟。否则就会出现ORA-01466错误。

[oracle@jumper oracle]$ sqlplus eygle/eygle
SQL*Plus: Release 9.2.0.4.0 - Production on Mon Dec 19 19:34:53 2005
Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.

Connected to:
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production
19:34:58 SQL> col myscn for 9999999999999999
19:34:58 SQL> variable myscn number;
19:34:58 SQL> create table eygle (c1 number);
Table created.
19:35:00 SQL> exec :myscn := dbms_flashback.get_system_change_number();
PL/SQL procedure successfully completed.
19:35:00 SQL> print myscn;
            MYSCN
-----------------
     107374274756
19:35:00 SQL> insert into eygle values(1);
1 row created.
19:35:00 SQL> commit;
Commit complete.
19:35:00 SQL> exec :myscn := dbms_flashback.get_system_change_number();
PL/SQL procedure successfully completed.
19:35:00 SQL> print myscn;
            MYSCN
-----------------
     107374274777
19:35:00 SQL> select * from eygle as of scn(:myscn);
select * from eygle as of scn(:myscn)
              *
ERROR at line 1:
ORA-01466: unable to read data - table definition has changed
19:41:10 SQL> select * from eygle as of scn(:myscn);
select * from eygle as of scn(:myscn)
              *
ERROR at line 1:
ORA-01466: unable to read data - table definition has changed
19:43:03 SQL> select dbms_flashback.get_system_change_number myscn from dual;
            MYSCN
-----------------
     107374274944
19:43:15 SQL> select * from eygle as of scn 107374274944;
        C1
----------
         1