The Oracle Database (commonly referred to as Oracle RDBMS or simply as Oracle) is a relational database management system (RDBMS) produced and marketed by Oracle Corporation. As of 2010 [update], Oracle has been a major presence in database computing for many years.
Users of the 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.If the Oracle database administrator has implemented Oracle RAC (Real Application Clusters), then multiple instances, usually on different servers, attach to a central storage array. This scenario offers advantages such as better performance, scalability and redundancy. However, support becomes more complex, and many sites do not use RAC. In version 10g, grid computing 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.
The Oracle RDBMS stores data logically in the form of tablespaces and physically in the form of data files (“datafiles”). Tablespaces can contain various types of memory segments, such as 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.
Oracle database management tracks 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.
The data dictionary comprises a set of tables and views that map the structure of the database. Oracle databases store information here about the logical and physical structure of the database. The data dictionary contains information such as: