Step 6 - using Datafiles
Learn Oracle - Datafiles
Datafiles are the operating system files that hold the data within the database. The
data is written to these files in an Oracle proprietary format that cannot be read by
programs other than the database server. Tempfiles are a special class of datafiles that
are associated only with temporary tablespaces.
Datafiles can be broken down into the following components:
Segments and extents
A segment contains a specific type of database object. For example, tables are
stored in data segments, and indexes are stored in index segments.
An extent is a contiguous set of data blocks (defined next) within a segment.
Oracle initially allocates an extent of a specified size for a segment, but if that
extent fills, then more extents can be allocated
Data blocks, also called database blocks, are the smallest unit of I/O to database
storage. An extent consists of several contiguous data blocks. The default database
block size is specified by a Oracle DBA at database creation or defaulted by Oracle.
After the database has been created, it is not possible to change the default block
size without re-creating the database. However, it is possible to create tablespace
with a block size different than the default block size.
This tablespace is used to store permanent user objects and data.
Like the TEMP tablespace, every database should have a
tablespace for permanent user data that is assigned to users.
Otherwise, their objects will be created in the SYSTEM tablespace,
and this is not good practice.
In the preconfigured database,
USERS is assigned the default tablespace, and space for all objects
created by non-system users comes from this tablespace. For
system users, the default permanent tablespace remains SYSTEM.
Click Datafiles. Your database datafiles are displayed. Select a datafile and click View
to see more attributes for the datafile, or select a tablespace link to see the attributes of
the owning tablespace.