What is a Database?
We already know that a database is a collection of data. And this data is stored in form of tables at logical level, and in the datafiles at the physical level. There are some other files as well like Redo log files, Control files, Initialization files which stores important information about the database.
What is an Instance?
To view or update data stored in tables/datafiles, Oracle must start a set of background processes, and must allocate some memory to be used during database operation. The background processes and memory allocated by Oracle together make up an Instance.
Oracle 12c For Dummies中关于
✓ An instance can exist without a database. Yes, it’s true. You can start an Oracle instance and not have it access any database files. Why would you do this?
• This is how you create a database. There’s no chicken-or-egg debate here. You first must start an Oracle instance; you create the database from within the instance.
• An Oracle feature called Automatic Storage Management uses an instance but isn’t associated with a database.
✓ A database can exist without an instance but would be useless. It’s just a bunch of magnetic blips on the hard drive.
✓ An instance can access only one database. When you start your instance, the next step is to mount that instance to a database. An instance can mount only one database at a time.
✓ You can set up multiple instances to access the same set of files or one database. Clustering is the basis for the Oracle Real Application Clusters feature. Many instances on several servers accessing one central data- base allows for scalability and high availability.