Thursday, September 14, 2006

Oracle database

An Oracle database consists of a collection of data managed by an Oracle database management system. Popular generic usage also refers to the DBMS management software, but not necessarily a specific database under its control.
One can refer to the Oracle database management system unambiguously as Oracle DBMS or (since it manages databases which have relational characteristics) as Oracle RDBMS.
Oracle Corporation itself blurs the very useful distinction between:
data managed by an Oracle RDBMS
an Oracle database, and
the Oracle RDBMS software itself
when it refers nowadays to the Oracle RDBMS (the software it sells for the purpose of managing databases) as the Oracle Database. The distinction between the managed data (the database) and the software which manages the data (the DBMS / RDBMS) relies, in Oracle's marketing literature, on the capitalisation of the word database.
Oracle Corporation produces and markets the Oracle DBMS, which many database applications use extensively on many popular computing platforms.
Larry Ellison and his friends and former co-workers Bob Miner and Ed Oates - who had started a consultancy called Software Development Laboratories (SDL) - developed the original Oracle DBMS software. They called their finished product Oracle after the code name of a CIA-funded project they had worked on while previously employed by Ampex.

Database structure

An Oracle database comprises an instance and data storage. The 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).
Oracle users refer to the server-side memory structure as the SGA (System Global Area). The SGA typically holds cache information like 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 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 blocksize can vary.
Oracle keeps track of its 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).
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.
Older Oracle database installations (pre-10g) traditionally came 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.)


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