其实就是临时restore point的思想，有了enable_at_time(scn)我们在执行flashback query或者flashback versions query时就不需要写as of 和versions between 子句了。

SQL> select * from tt;

ID NAME
---------- --------------------
1 a
2 e
4 d

SQL> select dbms_flashback.get_system_change_number from dual;

GET_SYSTEM_CHANGE_NUMBER
------------------------
8504779

SQL> exec dbms_flashback.enable_at_system_change_number(8504779);

PL/SQL procedure successfully completed.

SQL> select * from tt;

ID NAME
---------- --------------------
1 a
2 e
4 d

SQL> insert into tt values(5,'f');
insert into tt values(5,'f')
*
ERROR at line 1:
ORA-08182: operation not supported while in Flashback mode

SQL> exec dbms_flashback.disable;

PL/SQL procedure successfully completed.

SQL> insert into tt values(5,'f');

1 row created.

SQL> commit;

Commit complete.

SQL> exec dbms_flashback.enable_at_system_change_number(8504779);

PL/SQL procedure successfully completed.

SQL> select * from tt;

ID NAME
---------- --------------------
1 a
2 e
4 d

SQL> exec dbms_flashback.disable;

PL/SQL procedure successfully completed.

SQL> select * from tt;

ID NAME
---------- --------------------
1 a
2 e
4 d
5 f

SQL>
--================================
SQL> select systimestamp from dual;

SYSTIMESTAMP
---------------------------------------------------------------------------
28-JAN-13 08.52.55.875000 AM +08:00

SQL> insert into tt values(6,'g');

1 row created.

SQL> commit;

Commit complete.

SQL> exec dbms_flashback.enable_at_time(to_timestamp('28-JAN-13 08.52.55','DD-MON-YY HH24:MI:SS'));

PL/SQL procedure successfully completed.

SQL> select * from tt;

ID NAME
---------- --------------------
1 a
2 e
4 d
5 f

SQL> insert into tt values(7,'g');
insert into tt values(7,'g')
*
ERROR at line 1:
ORA-08182: operation not supported while in Flashback mode

SQL> exec dbms_flashback.disable;

PL/SQL procedure successfully completed.

SQL>