Databases Reference
In-Depth Information
The logical storage management of the database's data is independent of the
physical storage of the database's physical files on disk. This makes it possible
for changes to the physical structures to be transparent to the database user at the
logical level.
Tablespaces
A
is the highest level of logical objects in the database. A database
consists of one or more tablespaces. A tablespace will frequently group together
similar objects, such as tables, for a specific business area or a specific function.
A particular tablespace can be reorganized, backed up, and so forth with mini-
mal impact to other users whose data may be in other tablespaces.
All Oracle databases must have at least two tablespaces: the
tablespace
tablespace
A logical grouping of database objects,
usually to facilitate security, perfor-
mance, or the availability of database
objects such as tables and indexes. A
tablespace is composed of one or more
datafiles on disk.
SYSTEM
tablespace and the
SYSAUX
tablespace. Having more than just the
SYSTEM
and
SYSAUX
tablespaces is highly recommended when creating a database. In the
illustration of logical structures, you can see the
SYSTEM
tablespace, the
SYSAUX
tablespace, and two others. Oracle's Database Configuration Assistant, dis-
cussed later in this chapter, creates a total of six tablespaces for a default instal-
lation of Oracle 10
g
.
Segments
. A database segment is a type
of object that a user typically sees, such as a table. Tablespace 1 in the logical
structure illustration consists of three segments, which could be tables, indexes,
and so forth. It's important to note that this is the logical representation of these
objects; the physical representation of these objects in the operating system files
will most likely not resemble the logical representation.
A tablespace is further broken down into
segments
segment
A set of extents allocated for a single type
of object, such as a table.
Extents
. A segment
groups one or more extents allocated for a specific type of object in the data-
base. Segment 2 in the logical structure illustration consists of two extents.
Note that an extent cannot be shared between two segments. Also, a segment,
and subsequently an extent, cannot cross a tablespace boundary.
The next-lowest logical grouping in a database is the
extent
extent
A contiguous group of blocks allocated for
use as part of a table, index, and so forth.
Database Blocks
At the other end of the spectrum of logical objects is the
database block
(also
database block
The smallest unit of allocation in an Oracle
database. One or more database blocks
compose a database extent.
known as an
), the smallest unit of storage in an Oracle database.
Every database block in a tablespace has the same number of bytes. Starting with
Oracle9
Oracle block
i
, different tablespaces within a database can have database blocks with
