Oracle Instance Startup

Instance and database are normally considered to be the same things and there is no harm in doing so. Although technically both are different but without each other both are meaningless, therefore terms are used interchangeably. Database is just a dump of files that contain the data while instance provides a mean to access that data. Instance consists of some background processes and memory structures which are collectively called SGA or Shared Global Area. The process through which we make the data stored in the database available to users is knows as startup which is actually Instance startup.

There are several stages through which an Instance passes during startup. The reason for the split of Instance startup into stages is to perform certain operations at certain stages. For example most of the administrative tasks are performed in No mount or mount state. When the Instance is in mount or No mount state, it is unavailable to users and only DBA can access it. The initial stage of the Instance is idle. When we give the command startup the Instance first reads the spfile (Server Parameter File) and gets into No mount stage. Server Parameter File is a parameter file that contains the Name/Value pairs of parameters which are necessary for Instance startup. After that Instance reads the control files and gets into the mount state. Finally it reads the data files and gets into open state.

You can also start the Instance step by step. For example if you want to get the instance into mount state then you can issue the command
SQL>startup mount; instead of
SQL>startup
(The command startup will open the database.) After you have mounted your database then you can open it with the following command
SQL>alter database open;
One point worth mentioning here is that you cannot go through the states in reverse order. Means that you cannot mount a database when it is open and hence cannot bring it to No mount state when it is in mount state. If you want to do this then first shutdown the database and then bring it to the desired state.