Say one database has more than one instance. In such a case, is there any relation or proportion between instances and processors(cpu)?
For Oracle databases, only one instance can connect to a database at any given time. In Oracle-speak, the "database" is just the datafiles on disk, or the physical part. The "instance" is the memory structures which make the database up and running, or the logical part. Without both, the data cannot be accessed. Oracle will only let you start one instance to access a database. If you try to start another instance to access the same database, you will receive an error.
The exception to this is if you use Oracle Parallel Server (OPS). OPS changed its name to Real Application Clusters in Oracle 9i. This lets you start multiple instances on multiple machines all accessing the same database.
Any instance (OPS or not) will utilize all CPUs on the server.
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