The relational database management system (RDBMS) officially
called Oracle Database (and commonly referred to as Oracle
RDBMS or simply as Oracle) has become a major presence in
database computing. Oracle Corporation produces and markets
this software.
Larry Ellison and his friends and former co-workers Bob
Miner and Ed Oates started the consultancy Software
Development Laboratories (SDL) in 1977. SDL developed the
original version of the Oracle software. The name Oracle
comes from the code-name of a CIA-funded project Ellison had
worked on while previously employed by Ampex.
Many widespread computing platforms have come to use the
Oracle database software extensively.
An Oracle database system comprises at least one instance of
the application, along with data storage. An instance
comprises a set of operating-system processes and
memory-structures that interact with the storage. Typical
processes include PMON (the process monitor) and SMON (the
system monitor).
Users of Oracle databases refer to the server-side
memory-structure as the SGA (System Global Area). The SGA
typically holds cache information such as data-buffers, SQL
commands and user information. In addition to storage, the
database consists of online redo logs (which hold
transactional history). Processes can in turn archive the
online redo logs into archive logs (offline redo logs),
which provide the basis (if necessary) for data recovery and
for some forms of data replication.
The Oracle RDBMS stores data logically in the form of
tablespaces and physically in the form of data files.
Tablespaces can contain various types of memory segments;
for example, Data Segments, Index Segments etc. Segments in
turn comprise one or more extents. Extents comprise groups
of contiguous data blocks. Data blocks form the basic units
of data storage. At the physical level, data-files comprise
one or more data blocks, where the block size can vary
between data-files.
Oracle database management keeps track of its computer data
storage with the help of information stored in the SYSTEM
tablespace. The SYSTEM tablespace contains the data
dictionary — and often (by default) indexes and clusters. (A
data dictionary consists of a special collection of tables
that contains information about all user-objects in the
database). Since version 8i, the Oracle RDBMS also supports
"locally managed" tablespaces which can store space
management information in bitmaps in their own headers
rather than in the SYSTEM tablespace (as happens with the
default "dictionary-managed" tablespaces).
