Re: Basic Oracle Concepts

newbie wrote:

> 1) Every RUNNING database is associated with an Oracle instance. But is it
> possible to create > 1 databases in the harddisk while allowing the instance
> to select the specific one to be mounted during database startup? If yes,
> is it by specifying the init.ora to be used in 'STARTUP' ?

To add to Sybrand's comments. Keep in mind that a database in Oracle terminology refers to a PHYSICAL database - i.e. a collection of data files that are used as Oracle tablespaces, where these tablespaces are used to store all kinds of objects (tables, indexes, code, temp tables, etc).

Other products refer to a database as the LOGICAL entity. Where the database is a collection of related tables and indexes and stored procs and so on. In Oracle a logical database is called a schema.

As for your question, the converse is possible. Having multiple instances (aka database engines) for the same physical database. Thus 1 Database with N Instances. This is called RAC (Real Application Cluster) in Oracle.

With RAC, you can use various forms of logical partitioning of services, applications and data. For example, you can decide that RAC node 1 handles shipping and invoicing. RAC node 2 handles order entry. RAC node 3 handles marketing & sales analysis. Or decide that all services are available on each RAC node. Behind each RAC node sits the same (and *fully* shared) physical database. (RAC is a shared-everything db cluster architecture)

Thus the Oracle architecture is extremely flexible and allows you at the logical level all the flexibility and features that you will find in other commercial database products. And more. You can have 1000's of schemas (logical databases) in an Oracle physical database. That physical database can be serviced by a number of db engines. Each engine in turn can be allocated to service a specific set of logical databases and applications.

Flexibility and scalability are key features in Oracle.

--
Billy