The three steps to shutting down a database and its associated instance are:

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Close the database.
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Unmount the database.
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Shut down the instance.

Close a Database

When you close a database, Oracle writes all database data and recovery data in the SGA to the datafiles and redo log files, respectively. Next, Oracle closes all online datafiles and online redo log files. At this point, the database is closed and inaccessible for normal operations. The control files remain open after a database is closed but still mounted.



Close the Database by Terminating the Instance



In rare emergency situations, you can terminate the instance of an open database to close and completely shut down the database instantaneously. This process is fast, because the operation of writing all data in the buffers of the SGA to the datafiles and redo log files is skipped. The subsequent reopening of the database requires recovery, which Oracle performs automatically.

Un mount a Database

After the database is closed, Oracle un mounts the database to disassociate it from the instance. At this point, the instance remains in the memory of your computer.

After a database is un mounted, Oracle closes the control files of the database.

Shut Down an Instance

The final step in database shutdown is shutting down the instance. When you shut down an instance, the SGA is removed from memory and the background processes are terminated.

Abnormal Instance Shutdown

In unusual circumstances, shutdown of an instance might not occur cleanly; all memory structures might not be removed from memory or one of the background processes might not be terminated. When remnants of a previous instance exist, a subsequent instance startup most likely will fail. In such situations, the database administrator can force the new instance to start up by first removing the remnants of the previous instance and then starting a new instance, or by issuing a SHUTDOWN ABORT statement in Enterprise Manager.