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 data storage with the help of information stored in the SYSTEMM 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 SYSTEMM tablespace (as happens with the default "dictionary-managed" tablespaces).

If the Oracle database administrator has instituted Oracle RAC (Real Application Clusters), then multiple instances, usually on different servers, attach to a central storage array. This scenario offers numerous advantages, most importantly performance, scalability and redundancy. However, support becomes more complex, and many sites do not use RAC. In version 10g, grid computing has introduced shared resources where an instance can use (for example) CPU resources from another node (computer) in the grid.

The Oracle DBMS can store and execute stored procedures and functions within itself. PL/SQL (Oracle Corporation's proprietary procedural extension to SQL), or the object-oriented language Java can invoke such code objects and/or provide the programming structures for writing them.

Schemas

Oracle database conventions refer to defined groups of ownership (generally associated with a "username") as schemas.

Most Oracle database installations traditionally come with a default schema called SCOTT. After the installation process has set up the sample tables, the user can log into the database with the username scott and the password tiger. (The name of the SCOTT schema originated with Bruce Scott, one of the first employees at Oracle (then Software Development Laboratories), who had a cat named Tiger.)

Other default schemas include:

• SYS (essential core database structures and utilities)
• SYSTEM (additional core database structures and utilities, and privileged account)
• OUTLN (utilized to store metadata for stored outlines for query optimizer plan stability.
• BI, IX, HR, OE, PM, and SH (expanded sample schemas containing more data and structures than the older SCOTT schema)

Tablespaces

Default tablespaces include:

• SYSTEM (essential core database structures and utilities)
• SYSAUX (extra/extended data to supplement the SYSTEM schema)
• TEMP (temporary tablespace)
• UNDOTBS1 (undo tablespace)
• USERS (default users tablespace created by the Database Configuration Assistant - but replaceable by the DBA)

Memory architecture

System Global Area

Each Oracle instance uses a System Global Area or SGA ? a shared-memory area ? to store its data and control information.

Each Oracle instance allocates itself an SGA when it starts and de-allocates it at shut-down time. The information in the SGA consists of the following elements, each of which has a fixed size, established at instance startup:

• the database buffer cache: this stores the most recently-used data blocks. These blocks can contain modified data not yet written to disk (sometimes known as "dirty blocks"), unmodified blocks, or blocks written to disk since modification (sometimes known as clean blocks). Because the buffer cache keeps blocks based on a most-recently-used algorithm, the most active buffers stay in memory to reduce I/O and to improve performance.
• the redo log buffer: this stores redo entries ? a log of changes made to the database. The instance writes redo log buffers to the redo log as quickly and efficiently as possible. The redo log aids in instance recovery in the event of a system failure.
• the shared pool: this area of the SGA stores shared-memory structures such as shared SQL areas in the library cache and internal information in the data dictionary. An insufficient amount of memory allocated to the shared pool can cause performance degradation.

Library cache

The library cache stores shared SQL, caching the parse tree and the execution plan for every unique SQL statement.

If multiple applications issue the same SQL statement, each application can access the shared SQL area: this reduces the amount of memory needed and reduces the processing-time used for parsing and execution planning.

Data-dictionary cache

The data dictionary comprises a set of tables and views that map the structure of the database.

Oracle stores information here about the logical and physical structure of the database. The data dictionary contains information such as the following:

• User-information, such as user-privileges
• Integrity constraints defined for tables in the database
• Names and data-types of all columns in database tables
• Information on space allocated and used for schema objects

The Oracle instance frequently accesses the data dictionary in order to parse SQL statements. The operation of Oracle depends on ready access to the data dictionary: performance bottlenecks in the data dictionary affect all Oracle users. Because of this, database administrators should make sure that the data-dictionary cache has sufficient capacity to cache this data. Without enough memory for the data-dictionary cache, users see a severe performance-degradation. Allocating sufficient memory to the shared pool where the data-dictionary cache resides precludes these particular performance-problems.

Program Global Area

The Program Global Area or PGA memory area contains data and control information for the Oracle server processes.

The size and content of the PGA depends on the Oracle-server options installed. This area consists of the following components:

• stack-space: the memory that holds the session's variables, arrays, and so on.
• session-information: unless using the multithreaded server, the instance stores its session information in the PGA. (In a multithreaded server, the session information goes in the SGA.)
• private SQL-area: an area in the PGA holding information such as bind-variables and runtime buffers.
• sorting area: an area in the PGA holding information on sorts, hash-joins, etc.

History

Corporate and technical development

This article or section may contain a proseline.

Larry Ellison founded Software Development Laboratories in 1977. In 1979 SDL changed its company-name to Relational Software, Inc. (RSI) and introduced its product Oracle V2 as an early commercially-available relational database system. The version did not support transactions, but implemented the basic SQL functionality of queries and joins. (RSI never released a version 1 - instead calling the first version version 2 as a marketing gimmick.)

In 1983, RSI in its turn changed its name, becoming known as Oracle Corporation to align itself more closely with its flagship product. The company released Oracle version 3, which it had re-written using the C programming language and which supported COMMIT and ROLLBACK functionality for transactions. Version 3 extended platform support from the existing Digital VAX/VMS systems to include Unix environments.

In 1984, Oracle Corporation released Oracle version 4, which supported read-consistency.

From 1985, the Oracle RDBMS began supporting the client-server model, with networks becoming available in the mid-1980s. Oracle version 5.0 supported distributed queries.

In 1989, Oracle Corporation entered the application products market and developed its ERP product - Oracle Financials based on the Oracle relational database. Oracle RDBMS version 6 came out with support for PL/SQL embedded within Oracle Forms v3 (PLSQL could not be stored in the database in v6), row-level locking and hot backups.

In 1992, Oracle version 7 appeared with support for referential integrity, stored procedures and triggers.

In 1997, Oracle Corporation released version 8, which supported object-oriented development and multimedia applications.

In 1999, Oracle8i came out, aiming to operate more in tune with the needs of the Internet (The i in the name stands for "Internet"). The Oracle 8i database incorporated a native Java virtual machine (JVM).

In 2001, Oracle9i appeared with 400 new features, including the ability to read and write XML documents. 9i also provided an option for Oracle RAC, or "Real Application Clusters", a computer-cluster database, as a replacement for the Oracle Parallel Server (OPS) option.

In 2003, Oracle Corporation released Oracle Database 10g. The g stands for "grid"; emphasizing a marketing thrust of presenting 10g as "grid-computing ready".

In 2005, Oracle Database 10.2.0.1 ? also known as Oracle Database 10g Release 2 (10gR2) ? appeared.

In 2007, Oracle Corporation released Oracle Database 11g for the Linux operating system.

List of claimed firsts

Oracle Corporation claims to have provided:

• the first commercially-available SQL-based database (1979)
• the first database to support symmetric multiprocessing (SMP) (1983)
• the first distributed database (1986)
• the first database product tested to comply with the ANSI SQL standard (1993)
• the first 64-bit database (1995)
• the first web database (1997)
• the first database to incorporate a native JRE (1998)
• the first commercial RDBMS to become available on Linux (1998)
• the first database to support XML (1999)

Editions

Over and above the different versions of the Oracle database management software, Oracle Corporation subdivides its product into varying "editions" - apparently for marketing and license-tracking reasons.

• Standard Edition (SE) contains base database functionality. Oracle Corporation licenses this product on the basis of users or of processors, typically for servers running from one to four CPUs. If the number of CPUs exceeds 4 CPUs, the user must convert to an Enterprise license. SE has no memory limits, and can utilize clustering with Oracle RAC at no additional charge.
• Enterprise Edition (EE) includes more features than the 'Standard Edition', especially in the areas of performance and security. Oracle Corporation licenses this product on the basis of users or of processors, typically for servers running 4 or more CPUs. EE has no memory limits, and can utilize clustering using Oracle RAC software.
• Standard Edition One, introduced with Oracle 10g, has some additional feature-restrictions. Oracle Corporation markets it for use on systems with one or two CPUs. It has no memory limitations.
• Express Edition ('Oracle Database XE'), introduced in 2005, offers Oracle 10g free to distribute on Windows and Linux platforms (with a footprint of only 150 MB and restricted to the use of a single CPU, a maximum of 4 GB of user data and 1 GB of memory). Support for this version comes exclusively through on-line forums and not through Oracle support.
• Oracle Personal Edition provides the functionality of the "high end" Enterprise Edition but marketed to (and licensed for) single-user developers working on personal workstations.
• Oracle Database Lite, intended to run on mobile devices. The database, partially located on the mobile device, can synchronize with a server-based installation.
• TimesTen, a memory-resident database that has the ability to cache transactions and synchronize data with a centralized Oracle database server. It functions as a real-time infrastructure software product designed for low latency, high-volume data, event and transaction management
• BerkeleyDB, high-performance, embedded database library

Host platforms

Prior to releasing Oracle9i, Oracle Corporation ported the database engine to a wide variety of platforms. More recently, Oracle Corporation has consolidated on a smaller range of operating-system platforms.

Related software

For links to some of Oracle Corporation's software which integrates with Oracle databases, see the Oracle Corporation category and the Oracle software category pages.

Database "features"

Apart from the clearly-defined database options, Oracle databases may include many semi-autonomous software sub-systems, which Oracle Corporation sometimes refers to as "features". Such "features" may include (for example):

• Automatic Workload Repository (AWR), providing monitoring services to Oracle database installations from Oracle version 10. Prior to the release of Oracle version 10, the Statspack facility provided similar functionality.
• Data Aggregation and Consolidation
• Data Guard for high availability
• Flashback for selective data recovery and reconstruction
• iSQL*Plus, a web-browser-based interface to Oracle database DML
• Recovery Manager (rman) for database backup, restoration and recovery
• SQL*Plus, a command-line oriented interface to Oracle database DML and DDL

See also the category "Oracle software" (linked below)

Getting started

Users new to Oracle who need to get databases up and running may check out the online documentation, the Oracle Technology Network site, and the comp.databases.oracle Usenet discussion group. The Oracle Technet site offers downloads of full-featured evaluation software. Users who have Oracle support contracts should turn to Oracle's Metalink web site (password required). Users can also check the Oracle FAQ site before posting questions to forums, mailing lists, etc. They can also log on to http://asktom.oracle.com to post questions to and get answers from Tom Kyte, the author of several Oracle books including Expert One-On-One Oracle (ISBN 1-59059-525-4).

The Oracle RDBMS has had a reputation among novice users as difficult to install on Linux systems. Oracle Corporation has packaged recent versions for several popular Linux distributions in an attempt to minimize installation challenges beyond the level of technical expertise required to install a database server. ^{[citation needed]}

Oracle database in the market

Competition

In the market for relational databases, Oracle competes against commercial products such as IBM's DB2 UDB and Microsoft SQL Server. Oracle and IBM tend to battle for the mid-range market (especially on UNIX and Linux platforms), while Microsoft tends to compete in the low-end market (on Microsoft Windows platforms). However, since they share many of the same customers, Oracle and IBM tend to support each other's products in many middleware and application categories (for example, WebSphere, PeopleSoft, and Siebel Systems CRM), and IBM's hardware divisions work closely with Oracle on performance-optimizing server-technologies (for example, Linux on zSeries). The two companies have a relationship perhaps best described as "coopetition". Niche commercial competitors include Teradata (in data warehousing and business intelligence), Software AG's Adabas, Sybase, and IBM's Informix, among many others.

Increasingly, the Oracle database products compete against open-source relational database systems, particularly PostgreSQL, Firebird, and MySQL. Oracle acquired Innobase, supplier of the InnoDB codebase to MySQL, in part to compete better in the open source market. Database products developed on the basis of the open-source model generally cost significantly less to acquire than Oracle systems. EnterpriseDB, based on PostgreSQL, has recently made inroads by proclaiming that its product delivers Oracle compatibility features at a much lower price.

Recently there has been competition with SAP AG software and litigation from Oracle.