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Tablespaces in Oracle are equivalent to SQL Server Filegroups. They each are logical groupings of physical data files used to store related database objects, such as tables, indexes, etc. In Oracle 10g, two tablespaces are created when creating a database, by default: SYSTEM and SYSAUX. In SQL Server, one filegroup is created when creating a database, by default: PRIMARY. In both RDBMSes, you can create user-defined tablespaces and filegroups, in addition the the preceding mentioned. In Oracle 10g, you have the added benefit of being able to mark a tablespace as temporary, allowing tables added to be managed on a per-user-basis. For example, data added to a table in a temporary tablespace will only be modifiable by that user and that user only. There are other tablespace features, such as making them dictionary or locally managed. Oracle Blocks, Extents, and Segments & SQL Server Pages & Extents