Starting up Oracle database requires some important detail to note. Failure to follow the proper steps would end up to failed database startup.

Remember that we need to login as a user with SYSDBA privileges to startup an Oracle database which would enable us to for example do a – SQL>connect / as sysdba.

Oracle Database Startup Process Modes:

• Unmounted
• Mounted
• Open

Assuming that the Oracle database is completely shutdown and nothing is running. The first thing is to have the database in unmounted mode. At this stage the database instance should be up and running. An instance is a collection of background processes and memory structure that allows us to tap into our database.

The next step of the startup process is the mounted mode. Mount mode is the stage where an instance is actually associated with the database. The specific parameter in the initialization file which is responsible for this is the control_files. By the way, Oracle control files are small binary file which records physical structure of the database like database name and location of datafiles, redo log files. It is important as it is needed for mounting and recovery of an Oracle database. If for some reason the control file is missing or corrupted this particular stage the startup process will fail and consequently the database could not be opened.

The last stage of the Oracle database startup process is the open mode. Open mode is where the datafile and the redo logs are opened. This is the stage wherein the database is also available for everyone to use.