Oracle Database Foundations
An Oracle server consists of both a database and an instance. In Oracle terminology, database refers to only the physical files on disk. These are the files that store the data itself, the database state information in the control file, and the changes made to the data in the redo log files.
The term instance refers to the Oracle processes and memory structures that reside in the server’s memory and access an Oracle database on disk. One of the reasons for separating the concepts of a database and an instance is that a database may be shared by two or more different Oracle instances as part of an Oracle configuration that enhances the scalability, performance, and reliability of the Oracle server.
Database –The collection of all physical files on disk that are associated with a single Oracle instance.
It’s also important to differentiate between the logical and physical structures of the database. The logical structures represent components such as tables—what you normally see from a user’s point of view. The physical structures are the underlying storage methods on disk—the physical files that compose the database.
Instance -The collection of memory structures and Oracle background processes that operates against an Oracle database.
A logical grouping of database objects, usually to facilitate security, performance, or the availability of data base objects such as tables and indexes. A table space is composed of one or more data files on disk.
Once the Oracle software is installed on a server, you can create one or more database instances using a single copy of the Oracle software. The Database Configuration Assistant (DBCA) is Oracle’s GUI tool for creating, modifying, and deleting databases.
For the Microsoft Windows platform, each Oracle instance requires at least 256MB of memory, plus 8GB of disk space for a fairly complete installation of Oracle Enterprise Edition. Oracle strongly recommends at least 512MB of memory. The amount of disk space needed for the datafiles depends on the application’s data needs, but one of Oracle’s starter databases uses approximately 1.5GB of disk space.