Oracle Logical Storage Structures

Oracle Logical Storage Structures
The datafiles in an Oracle database are grouped together into one or more tablespaces. Within
each tablespace, the logical database structures, such as tables and indexes, are segments that are
further subdivided into extents and blocks. This logical subdivision of storage allows Oracle to
have more efficient control over disk space usage. Figure 1-2 shows the relationship between the
logical storage structures in a database.
Tablespaces

An Oracle tablespace consists of one or more datafiles; a datafile can be a part of one and only
one tablespace. For an installation of Oracle 11g, a minimum of two tablespaces are created: the
SYSTEM tablespace and the SYSAUX tablespace; a default installation of Oracle 11g creates six
tablespaces (see the appendix “Installation and Configuration” for sample Oracle 11g installations).
Oracle 11g allows you to create a special kind of tablespace called a bigfile tablespace, which
can be as large as 128TB (terabytes). Using bigfiles makes tablespace management completely
transparent to the DBA; in other words, the DBA can manage the tablespace as a unit without
worrying about the size and structure of the underlying datafiles.
Using Oracle Managed Files (OMF) can make tablespace datafile management even easier.
With OMF, the DBA specifies one or more locations in the file system where datafiles, control
files, and redo log files will reside, and Oracle automatically handles the naming and management
of these files. We discuss OMF in more detail in Chapter 4.
If a tablespace is temporary, the tablespace itself is permanent; only the segments saved in
the tablespace are temporary. A temporary tablespace can be used for sorting operations and for
tables that exist only for the duration of the user’s session. Dedicating a tablespace for these kinds