Database vs instance

Question

Can anyone tell me the difference between a database and an instance?
I have heard the terms being used interchangably without knowing the actual meaning of the two.
Thanks.

Answer 1

It's a really messy situation, the way people make use of it. The context of the discussion is usually the only way to know what someone is actually referring to.
I've seen an Oracle instance defined as a set of certain processes running on a system. I think this includes such things as the database writer (DBWR), log writer (LGWR), and recovery processes (SMON and PMON). The OCP exam preparation book for the DBA exams has this definition in detail.
I've always considered a database to be a collection of tables that make up a specific system or serve a particular purpose, but the Oracle definition refers to a database as certain files that it controls.
The two terms are frequently used as though they are synonymous. When an instance is started, the background processes I mentioned above start running on the sytem. Then, when a database is opened, the control files associated with a particular database are opened and the datafiles associated with it are verified.
At the time a database is created with the CREATE DATABASE statement, certain datafiles and control files are defined and created. Then these files always make up the database that you've defined.
As an example, I could have a hardware system named SUPERCPU. When I start the instance named SUPERDB, some processes start running on SUPERCPU. There are a few files associated with this instance, which we generally don't deal with, except for the intialization parameters. Then when I open a database named PAYROLL, the files that were created with the CREATE DATABASE PAYROLL statement are opened and accessed by the processes running for the instance SUPERDB.
On most hardware platforms it is possible to have multiple instances running simultaneously. Each instance gets its own set of processes. It is possible to issue multiple CREATE DATABASE statements for a single instance, so you could have multiple databases running on a single instance. Most frequently, though I've seen just one database associated with one instance; if there's a need for another database, people will start another instance. Then, finally, of course, you could create a set of tables for your payroll application and call that your payroll database. You could also create a set of tables for your accounts receivable application and call that your AR database. These two sets of tables could reside in the same Oracle database.
The only way I've been able to keep these things straight as I move from installation to installation is to take them in context. Using the above example, if someone refers to SUPERDB, then they probably mean the instance running on SUPERCPU. If someone refers to the payroll database, they probably mean either a specific Oracle database or certain tables within some Oracle database.
As you can see, this whole issue is confusing. Take a look at some of the Oracle DBA books for the real definitions, then apply what you know to the context of the discussion.
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