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In Oracle terms, a "database" is the collection of
datafiles. An "instance" is the allocated memory and
running processes. In Oracle there is a one to one
correspondence between instance and database. One
instance can attach to only one database. And one
database can only have one instance attached to it.
People coming from a SQL Server background have
difficulties with this because "database" means
something totally different to SQL Server. The closest
thing to a SQL Server database in Oracle is the
"tablespace". Just like SQL Server can have multiple
databases, Oracle can have multiple tablespaces.
If you want to create a new instance on your server,
then you certainly can. But keep in mind that it needs
its own database to attach to. You can't attach the
second instance to the same database.
The exception to the above rule is to run an Oracle
add-on product called Real Application Clusters, or
RAC. Before Oracle 9i, this product was called Oracle
Parallel Server. RAC lets you attach multiple
instances to one database. But you'll want to put each
instance on a separate server connecting to the
database on a shared disk subsystem.
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