Re: Ora10g installed...now what ? :-)

On Wed, 16 Aug 2006 20:15:30 +0200, Fabio Cannavaro wrote:

>
> Can you explain me please in simple terms the difference from "database"
> term in sql server 2000 and Oracle?

Database, in SQL Server terms, is something you 'open' to get at a set of data. Usually it consists of a set of tables, views, etc. The information, or metadata, about a database is stored in the master catalog ...

Database, in Oracle terms, is simply a set of files. Some of these files are used as tablespaces - where tables will eventually be stored.

Oracle schemas are somewhat similar to SQL Server databases. The SYS schema is equivalent to the master catalog/master database. There are dozens of Oracle supplied schemas. You can also create your own - often a schema matches to an application.

>
> - Does an instance of Oracle contain one or more databases ?

No, but that doesn't really matter because you can have more than one schema in an Oracle database.

All schemas are open once the database is up - and all are subject to security. Users need to be given explicit access to objects within a schema and Oracle uses the prinipal of least privilege ... if you have not been given access, you can assume you don't have access.

> - What is exactly a "schema" ?

It is really just a named set of metadata, that is, a collection of tables, views, etc. The namespace of the schema happens to match the userid called the owner. (In many ways, it is effectively equivalent to a SQL Server database.)

> - Is a tablespace like a database ? What is the difference from
> tablespace of Ora and database of Sql server?

A tablespace is a place for storing 'storables', such as tables and indexes. These 'storables' are called segments.

A convenient analogy is 'if a table is Oracle's virtual file, then a tablespace is Oracle virtual disk':
- Just like your operating system can have access to many disks; Oracle   can have many tablespaces.
- A file can reside only on one disk; a segment can reside in only one   tablespace.
- Having multiple disks can provide administrative and security benefits   (user x can not store a file on disk y); similarily, multiple   tablespaces can give the competent DBA administrative and security   benefits.

A table belongs to a schema and is stored in a tablespace. Any number of tables (and/or indexes) may be stored in a tablespace.

-- 
Hans Forbrich   (mailto: Fuzzy.GreyBeard_at_gmail.com)   
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