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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. , Oracle has been a major presence in database computing for many years. Larry Ellison and his friends and 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 at least one instance of the application, along with data storage. An instance—identified persistently by an instantiation number (or activation id: SYS.V_$DATABASE.ACTIVATION#)—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 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. 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 provide the basis (if necessary) for data recovery and for some forms of data replication. 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 10''g'', 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. === Storage === 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. Since version 8''i'', 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). ==== Disk files ==== At the physical level, datafiles comprise one or more data blocks, where the block size can vary between datafiles. Datafiles can occupy pre-allocated space in the file system of a computer server, utilize raw disk directly, or exist within ASM logical volumes. ===== Control files ===== The following parameters govern the size of the control files: * maxlogfiles * maxlogmembers * maxloghistory * maxinstances * control_file_record_keep_time ===Database schema=== Oracle database conventions refer to defined groups of object ownership (generally associated with a "username") as ''schemas''. Most Oracle database installations 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. Oracle Corporation has de-emphasized the use of the SCOTT schema, as it uses few of the features of the more recent releases of Oracle. Most examples supplied by Oracle Corporation reference the default HR or OE schemas. 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 stable query-optimizer execution plans. * BI, IX, HR, OE, PM, and SH (expanded sample schemas containing more data and structures than the older SCOTT schema). ====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 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 databases store information here about the logical and physical structure of the database. The data dictionary contains information such as: * user information, such as user privileges * integrity constraints defined for tables in the database * names and datatypes 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 of an Oracle instance contains data and control-information for Oracle's 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 which holds information such as bind-variables and runtime-buffers. * sorting area: an area in the PGA which holds information on sorts, hash-joins, etc. ====Dynamic performance views==== The dynamic performance views (also known as "fixed views") within an Oracle database present information from virtual tables (X$ tables) built on the basis of database memory. Database users can access the V$ views (named after the prefix of their synonyms) to obtain information on database structures and performance. ===Process architectures=== ====Oracle processes==== The Oracle RDBMS typically relies on a group of processes running simultaneously in the background and interacting to monitor and expedite database operations. Typical operating groups might include some of the following individual processes—(shown along with their abbreviated nomenclature): * archiver processes (ARCn) * checkpoint process (CKPT) *REQUIRED* * coordinator-of-job-queues process (CJQn): dynamically spawns slave processes for job-queues * database writer processes (DBWn) *REQUIRED* * dispatcher processes (Dnnn): multiplex server-processes on behalf of users * memory-manager process (MMAN): used for internal database tasks such as Automatic Shared Memory Management * log-writer process (LGWR) *REQUIRED* * log-write network-server (LNSn): transmits redo logs in Data Guard environments * logical standby coordinator process (LSP0): controls Data Guard log-application * media-recovery process (MRP): detached recovery-server process * memory-monitor process (MMON): process for automatic problem-detection, self-tuning and statistics-gathering * memory-monitor light process (MMNL): gathers and stores Automatic Workload Repository (AWR) data * mmon slaves (Mnnnn—M0000, M0001, etc): background slaves of the MMON process * process-monitor process (PMON) *REQUIRED* * process-spawner (PSP0): spawns Oracle processes * queue-monitor processes (QMNn) * recoverer process (RECO) * remote file-server process (RFS) * shared server processes (Snnn): serve client-requests * system monitor process (SMON) *REQUIRED* ====User processes, connections and sessions==== Oracle Database terminology distinguishes different computer-science terms in describing how end-users interact with the database: * user processes involve the invocation of application software * a connection refers to the pathway linking a user process to an Oracle instance * sessions consist of specific connections to an Oracle instance. Each session within an instance has a session identifier or "SID" (distinct from the system-identifier SID). ===Concurrency and locking=== Oracle databases control simultaneous access to data resources with locks (alternatively documented as "enqueues" ). The databases also utilize "latches" -- low-level serialization mechanisms to protect shared data structures in the System Global Area. ===Configuration=== Database administrators control many of the tunable variations in an Oracle instance by means of values in a parameter file. This file in its ASCII default form ("pfile") normally has a name of the format init.ora. The default binary equivalent server paramater file ("spfile") (dynamically reconfigurable to some extent) defaults to the format spfile.ora. Within an SQL-based environment, the views V$PARAMETER and V$SPPARAMETER give access to reading parameter values. ==Internationalization== Oracle Database software comes in 63 language-versions (including regional variations such as American English and British English). Variations between versions cover the names of days and months, abbreviations, time-symbols such as A.M. and A.D., and sorting. Oracle Corporation has translated Oracle Database error-messages into Arabic, Catalan, Chinese, Czech, Danish, Dutch, English, Finnish, French, German, Greek, Hebrew, Hungarian, Italian, Japanese, Korean, Norwegian, Polish, Portuguese, Romanian, Russian, Slovak, Spanish, Swedish, Thai and Turkish. Oracle Corporation provides database developers with tools and mechanisms for producing internationalized database applications: referred to internally as "Globalization". ==History== ===Corporate/technical timeline=== * 1977: Larry Ellison and friends founded Software Development Laboratories. * 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.) * 1982: RSI in its turn changed its name, becoming known as "Oracle Corporation", to align itself more closely with its flagship product. * 1983: 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.