When you start the Oracle instance, you typically start it such that the state of the database is OPEN and ready for user connections. However, there are situations in which you may want to start the instance with the database in the MOUNTED state, but not open. An instance can also be started without the database either mounted or open. Thus, there are three stages to starting an instance:
You start the instance using one of the following methods:
Using the SQL*Plus STARTUP command. See "Shutting Down and Starting Up Using SQL*Plus".
On Microsoft Windows, using the Services program in Control Panel to start the Oracle Database services. See "Shutting Down and Starting Up Using the Windows Services Program".
The instance reads the initialization parameter file, allocates System Global Area (SGA) memory, and starts the background processes.
If you mount the database, then the Oracle instance opens the control file for the database, but does not open the data files. The database is now considered to be in the MOUNT state. This state enables you to perform certain administrative functions that cannot be performed when other users are accessing the database. An example of such a function is enabling or disabling the archiving of online redo log files. See "About Archived Redo Log Files" for information about online redo log file archiving.
If you open the database, then, after reading the parameter file and control file, the online redo log files and data files for the database are also opened. The state of the database is now OPEN and user access to the data is available.
The default startup mode for the database (OPEN) completes the three stages in sequence. Unless you explicitly specify otherwise, the instance is started, the database is mounted, and then the database is opened.