Databases Reference
In-Depth Information
Oracle Components Overview
An Oracle server consists of both a database and an instance. In Oracle termi-
nology,
database
The collection of all physical files on disk
that are associated with a single Oracle
instance.
refers to only the physical files on disk. These are the files that
store the data itself, the database state information in the control file, and the
changes made to the data in the redo log files. The term
database
refers to the
Oracle processes and memory structures that reside in the server's memory and
access an Oracle database on disk. One of the reasons for separating the concepts
of a database and an instance is that a database may be shared by two or more
different Oracle instances as part of an Oracle configuration that enhances the
scalability, performance, and reliability of the Oracle server.
instance
It's also important to differentiate between the logical and physical structures of
the database. The logical structures represent components such as tables—what
you normally see from a user's point of view. The physical structures are the under-
lying storage methods on disk—the physical files that compose the database.
instance
The collection of memory structures and
Oracle background processes that oper-
ates against an Oracle database.
Logical Storage Structures
The Oracle database is divided into increasingly smaller logical units to manage,
store, and retrieve data efficiently and quickly. The illustration below shows the
relationships between the
logical structures
Structures in an Oracle database that a
database user would see, such as a
table, as opposed to the underlying phys-
ical structures at the datafile level.
logical structures
of the database: tablespaces, seg-
ments, extents, and blocks.
Database
SYSTEM
tablespace
SYSAUX
tablespace
Tablespace
1
Tablespace
2
Segment
Extent 1
Tablespace
Segment 1
Segment 2
Extent 2
Segment 3
Blocks
