Physical and Logical Storage Structures

The Oracle RDBMS is comprised of tablespaces, which in turn are comprised of data files. Tablespace data files are formatted into internal units termed blocks. The block size, set by the DBA when the Oracle database is first created, can range from 512 to 8192 bytes. When an object is created in an Oracle tablespace, the user specifies its space in units called extents (initial extent, next extent, min extents, and max extents). An Oracle extent varies in size and must contain a chain of at least five contiguous blocks.

Microsoft SQL Server uses filegroups at the database level to control the physical placement of tables and indexes. Filegroups are logical containers of one or more files, and data contained within a filegroup is proportionally filled across all files belonging to the filegroup.

If filegroups are not defined and used, database objects are placed in a default filegroup that is implicitly defined during the creation of a database. Filegroups allow you to:

SQL Server formats files into internal units called pages. The page size is fixed at 8192 bytes (8 KB). Pages are organized into extents that are fixed in size at 8 contiguous pages. When a table or index is created in a SQL Server database, it is automatically allocated one page. This allows for more efficient storage of smaller tables and indexes when compared to allocating an entire extent.