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Oracle Database (commonly referred to as Oracle RDBMS or simply as Oracle), a relational database management system (RDBMS) software product released by Oracle Corporation, has become a major feature of database computing. Many widespread computing platforms have come to use the Oracle database software extensively. Physical and logical structuring 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). 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.) The scott schema has fallen out of favor as it uses so few of the features of a modern release of oracle. Most recent examples reference the default hr or oe schema. 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)