How to pause an Oracle database

Sometimes it is useful to pause a database, and let it finish the running jobs at a later time.
Particularly in environments with shared resources contention may occur. Important databases need to be prioritized while less important ones should be suspended.

For this purpose, you can use the "suspend" command:

alter system suspend;

Alert log:

Mon Oct 05 10:14:49 2015
Suspending database after alter system suspend command

The database_status field in v$instance will reflect the new state of the database:

SQL> select database_status from v$instance;

 DATABASE_STATUS
-----------------

SUSPENDED

When you are ready to resume database activity, issue the following command:

alter system resume;

More info: Oracle Documentation

Two different ways to retreive the current System Change Number

Use either of the following methods:

SQL> col current_scn format 9999999999999
SQL> select current_scn from v$database;

   CURRENT_SCN
--------------
 6525378448555

SQL> col GET_SYSTEM_CHANGE_NUMBER format 9999999999999
SQL> select dbms_flashback.get_system_change_number from dual;

GET_SYSTEM_CHANGE_NUMBER
------------------------
           6525378448442

Oracle officially recommends tht if your goal is to obtain an SCN from within an application, then you should use the dbms_flashback.get_system_change_number function.