Introduction to an Oracle Instance
Connection with Administrator Privileges
  When you connect with SYSDBA privileges, you are in the schema owned by SYS. When you connect as SYSOPER, you are in the public schema. SYSOPER privileges are a subset of SYSDBA privileges.

Initialization Parameter Files
Most initialization parameters belong to one of the following groups:
  1.Parameters that name things, such as files
  2.Parameters that set limits, such as maximums
  3.Parameters that affect capacity, such as the size of the SGA, which are called variable parameters

Among other things, the initialization parameters tell Oracle:
  1.The name of the database for which to start up an instance
  2.How much memory to use for memory structures in the SGA
  3.What to do with filled online redo log files
  4.The names and locations of the database control files
  5.The names of undo tablespaces or private rollback segments in the database

Instance and Database Startup
How a Database Is Mounted with Real Application Clusters
  If Oracle allows multiple instances to mount the same database concurrently, then the database administrator can use the initialization parameter CLUSTER_DATABASE to make the database available to multiple instances. The default value of the CLUSTER_DATABASE parameter is false. Versions of Oracle that do not support Real Application Clusters only allow CLUSTER_DATABASE to be false.

  If CLUSTER_DATABASE is false for the first instance that mounts a database, then only that instance can mount the database. If CLUSTER_DATABASE is set to true on the first instance, then other instances can mount the database if their CLUSTER_DATABASE parameters are set to true. The number of instances that can mount the database is subject to a predetermined maximum, which you can specify when creating the database.

Open a Database in Read-Only Mode
  Read-only mode restricts database access to read-only transactions, which cannot write to the datafiles or to the redo log files.

  Disk writes to other files, such as control files, operating system audit trails, trace files, and alert files, can continue in read-only mode. Temporary tablespaces for sort operations are not affected by the database being open in read-only mode. However, you cannot take permanent tablespaces offline while a database is open in read-only mode. Also, job queues are not available in read-only mode.

  Read-only mode does not restrict database recovery or operations that change the database's state without generating redo data. For example, in read-only mode:
  1.Datafiles can be taken offline and online
  2.Offline datafiles and tablespaces can be recovered
  3.The control file remains available for updates about the state of the database
One useful application of read-only mode is that standby databases can function as temporary reporting databases.