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Oracle Architecture...
Database
- Disk-based files that contain system and user data
Instance
- Processes
- Memory
- System Global Area – Shared memory including buffer cache
- Cache for recently used data blocks
- Many more memory areas…
- Facilitates access to the database
Can not access database unless instance is started
Notes:
Oracle makes a distinction between database files that contain database data and the memory and process required to access those files. A database is the disk-based files that contain the system and user data. An instance is the active processes and memory areas that facilitate access the the database.
Instance and database each are assigned a name. They often have the same name (but not always true; in a Real Application Cluster environment, mulitple instances can access the same database - the instances will not have the exact same name.)
The instance name is called the System Identifier (SID). It is stored in the environment variable $ORACLE_SID. It can also been seen by querying the dynamic performance table v$instance. When you connect to the database with the CONNECT command, the current $ORACLE_SID (or the database alias explicitly specified in the CONNECT command) determines which instance you connect to and thus what database you are connected to. You cannot be connected to more than one database at a time.
The database name is assigned with the CREATE DATABASE command and is stored in a control file. Query v$database to see the database name.