About Data Files

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About Data Files
Data files are the operating system files that store the data within the database. The
data is written to these files in an Oracle proprietary format that cannot be read by
other programs. Tempfiles are a special class of data files that are associated only with
temporary tablespaces.
Data files can be broken down into the following components:
■ Segment
A segment contains a specific type of database object. For example, a table is
stored in a table segment, and an index is stored in an index segment. A data file
can contain many segments.
An extent is a contiguous set of data blocks within a segment. Oracle Database
allocates space for segments in units of one extent. When the existing extents of a
segment are full, the database allocates another extent for that segment.
■ Data block
A data block, also called a database block, is the smallest unit of I/O to database
storage. An extent consists of several contiguous data blocks. The database uses a
default block size at database creation.
After the database has been created, it is not possible to change the default block
size without re-creating the database. It is possible, however, to create a tablespace
with a block size different than the default block size.
Segments, extents, and data blocks are all logical structures. Only Oracle Database can
determine how many data blocks are in a file. The operating system recognizes only
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