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The Oracle Database (usually referred to as Oracle RDBMS or just
as Oracle) is an object-relational database management system (ORDBMS) produced
and marketed by Oracle Corporation.
Larry Ellison and his friends, 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
Physical and logical
structures
An Oracle database system—identified by an alphanumeric
system identifier or SID—comprises as a minimum one instance of the application,
along with data storage. An instance—recognized persistently by an
instantiation number (or activation id: SYS.V_$DATABASE.ACTIVATION#)—comprises
a set of operating-system processes and memory-structures that act together
with the storage. Typical processes comprise PMON (the process monitor) and
SMON (the system monitor).
Users of the Oracle databases turn to the server-side
memory-structure as the SGA (System Global Area). The SGA characteristically
holds cache information such as data-buffers, SQL commands, and user
information. In addition to storage, the database consists of online redo logs
(or logs), which hold transactional history. Processes can in turn archive the
online redo logs into archive logs (offline redo logs), which give the basis (if
essential) for data recovery and for some forms of data replication.
If the Oracle database administrator has employed Oracle RAC
(Real Application Clusters), then multiple cases in points, typically on
different servers, attach to a central storage array. This scenario proposes
advantages for example better performance, scalability and redundancy. On the
other hand, support becomes more complex and many sites do not use RAC. In
version 10g, grid computing commenced shared resources where an instance can
use (for example) CPU resources from one more node (computer) in the grid. The
Oracle DBMS can store and perform stored processes and functions within it.
Storage
The Oracle RDBMS amasses data logically in the form of table
spaces and physically in the form of data files ("data files").Table
spaces can contain a range of sorts of memory segments, for instance Data
Segments, Index Segments, etc. parts in turn comprise one or more extents.
Extents include sets of contiguous data blocks. Data blocks from the basic
units of data storage.
There is also a partitioning attribute available on newer
versions of the database, which permits tables to be partitioned based on dissimilar
set of keys. Particular partitions can then be with no trouble added or dropped
to help manage big data sets.
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