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

此时SMON_SCN_TIME中的信息如下:

19:45:11 SQL> select * from (select * from smon_scn_time order by time_dp desc) where rownum <5;

    THREAD    TIME_MP TIME_DP                SCN_WRP    SCN_BAS
---------- ---------- ------------------- ---------- ----------
         1 1134992658 2005-12-19 19:44:18         25      92565
         1 1134992351 2005-12-19 19:39:11         25      92463
         1 1134992042 2005-12-19 19:34:04         25      92256
         1 1134991731 2005-12-19 19:28:53         25      92149

在Oracle10g中，SMON_SCN_TIME表每6秒被更新一次。所以这个Flashback时间被缩小为6秒。

[oracle@danaly ~]$ sqlplus eygle/eygle

SQL*Plus: Release 10.2.0.1.0 - Production on Mon Dec 19 19:37:19 2005

Copyright (c) 1982, 2005, Oracle.  All rights reserved.


Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, Oracle Label Security, OLAP and Data Mining Scoring Engine options

19:37:22 SQL> col myscn for 9999999999999999
19:37:22 SQL> variable myscn number;
19:37:22 SQL> create table eygle (c1 number);

Table created.

19:37:22 SQL> exec :myscn := dbms_flashback.get_system_change_number();

PL/SQL procedure successfully completed.

19:37:22 SQL> print myscn;

            MYSCN
-----------------
      16011230122

19:37:22 SQL> insert into eygle values(1);

1 row created.

19:37:22 SQL> commit;

Commit complete.

19:37:22 SQL> exec :myscn := dbms_flashback.get_system_change_number();

PL/SQL procedure successfully completed.

19:37:22 SQL> print myscn;

            MYSCN
-----------------
      16011230124

19:37:22 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


19:37:22 SQL> select scn_to_timestamp(16011230124) from dual;

SCN_TO_TIMESTAMP(16011230124)
---------------------------------------------------------------------------
19-DEC-05 07.37.21.000000000 PM

19:38:50 SQL> select timestamp_to_scn('19-DEC-05 07.37.27.000000000 PM') MYSCN from dual;

            MYSCN
-----------------
      16011230125

19:39:02 SQL> select * from eygle as of scn 16011230125;

        C1
----------
         1

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what is SMON_SCN_TIME ?

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