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"oracle database"
  From Wikipedia:
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'''Oracle Database''' (commonly referred to as '''Oracle RDBMS''' or simply as '''Oracle''') is an object-relational database management system produced and marketed by Oracle Corporation.

Larry Ellison and his two friends and former co-workers, Bob Miner and Ed Oates, started a consultancy called 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 formerly 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). Oracle documentation can refer to an active database instance as a "shared memory realm".

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 (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 for data recovery and for the physical-standby forms of data replication using Oracle Data Guard.

The Oracle RAC (Real Application Clusters) option uses multiple instances attached to a central storage array. In version 10''g'', grid computing introduced shared resources where an instance can use CPU resources from another node in the grid. The advantage of Oracle RAC is that the resources on both nodes are used by the database, and each node uses its own memory and CPU. Information is shared between nodes through the interconnect—the virtual private network.

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.

A DBA can impose maximum quotas on storage per user within each tablespace.

====Partitioning====
The partitioning feature was introduced in Oracle 8. This allows the partitioning of tables based on different set of keys. Specific partitions can then be added or dropped to help manage large data sets.

====Monitoring====
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, 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 that store space management information in bitmaps in their own headers rather than in the SYSTEM tablespace (as happens with the default "dictionary-managed" tablespaces). Version 10''g'' and later introduced the SYSAUX tablespace, which contains some of the tables formerly stored in the SYSTEM tablespace, along with objects for other tools such as OEM, which previously required its own tablespace.

==== Disk files ====


Disk files primarily represent one of the following structures:

* Data and index files: These files provide the physical storage of data, which can consist of the data-dictionary data (associated to the tablespace SYSTEM), user data, or index data. These files can be managed manually or managed by Oracle itself. Note that a datafile has to belong to exactly one tablespace, whereas a tablespace can consist of multiple datafiles.
* Redo log files, consisting of all changes to the database, used to recover from an instance failure. Note that often a database will store these files multiple times, for extra security in case of disk failure. The identical redo log files are said to belong to the same group.
* Undo files: These special datafiles, which can only contain undo information, aid in recovery, rollbacks, and read-consistency.
* Archive log files: These files, copies of the redo log files, are usually stored at different locations. They are necessary (for example) when applying changes to a standby database, or when performing recovery after a media failure. It is possible to archive to multiple locations.
* Tempfiles: These special datafiles serve exclusively for temporary storage data (used for example for large sorts or for global temporary tables)
* Control file, necessary for database startup. "A binary file that records the physical structure of a database and contains the names and locations of redo log files, the time stamp of the database creation, the current log sequence number, checkpoint information, and so on."

At the physical level, data files comprise one or more data blocks, where the block size can vary between data files.

Data files can occupy pre-allocated space in the file system of a computer server, utilize raw disk directly, or exist within ASM logical volumes.

Database schema
Most Oracle database installations traditionally came with a default schema called SCOTT. After the installation process sets up 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 now de-emphasizes the SCOTT schema, as it uses few features of more recent Oracle releases. 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:
*Datafiles

Every Oracle database has one or more physical datafiles, which contain all the database data. The data of logical database structures, such as tables and indexes, is physically stored in the datafiles allocated for a database.

Datafiles have the following characteristics:

* One or more datafiles form a logical unit of database storage called a tablespace.
* A datafile can be associated with only one tablespace.
* Datafiles can be defined to extend automatically when they are full.

Data in a datafile is read, as needed, during normal database operation and stored in the memory cache of Oracle Database. For example, if a user wants to access some data in a table of a database, and if the requested information is not already in the memory cache for the database, then it is read from the appropriate datafiles and stored in memory.

Modified or new data is not necessarily written to a datafile immediately. To reduce the amount of disk access and to increase performance, data is pooled in memory and written to the appropriate datafiles all at once.

* 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.
* the Large pool Optional area that provides large memory allocations for certain large processes, such as Oracle backup and recovery operations, and I/O server processes
* Database buffer cache: Caches blocks of data retrieved from the database
* KEEP ''buffer pool'': A specialized type of database buffer cache that is tuned to retain blocks of data in memory for long periods of time
* RECYCLE buffer pool: A specialized type of database buffer cache that is tuned to recycle or remove block from memory quickly
* nK buffer cache: One of several specialized database buffer caches designed to hold block sizes different from the default database block size
* Java pool:Used for all session-specific Java code and data in the Java Virtual Machine (JVM)
* Streams pool: Used by Oracle Streams to store information required by capture and apply
When you start the instance by using Enterprise Manager or SQL*Plus, the amount of memory allocated for the SGA is displayed.

====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 to parse SQL statements. Oracle operation depends on ready access to the data dictionary—performance bottlenecks in the data dictionary affect all Oracle users. Because of this, database administrators must 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 this particular performance problem.

====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 that holds information such as bind-variables and runtime-buffers
* sorting area: an area in the PGA that holds information on sorts, hash-joins, etc.

DBAs can monitor PGA usage via the system view.

====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 environments might include - temporarily or permanently - some of the following individual processes (shown along with their abbreviated nomenclature):

* advanced queueing processes (Qnnn)
* 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*
* Data Pump master process (DMnn)

* Data Pump worker processes (DWnn)
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