The Oracle RDBMS Physical and logical structures Oracle Database administrators

The Oracle Database (commonly referred to as Oracle RDBMS or simply Oracle) is a relational database management system (RDBMS) produced and marketed by Oracle Corporation. As of 2009, Oracle remains a major presence in database computing.

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 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.

Storage

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, 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 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).

Control files

Size of control files The size of the control files is governed by the following parameters:
* 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 recent 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
Main article: System Global Area

Each Oracle instance uses a System Global Area or SGA—a shared-memory area—to store its data and control-information.[9]

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 database buffer cache: this stores the most recently-used data blocks. These blocks can contain modified data not yet written to disk (sometimes known as "dirty blocks"), unmodified blocks, or blocks written to disk since modification (sometimes known as clean blocks). Because the buffer cache keeps blocks based on a most-recently-used algorithm, the most active buffers stay in memory to reduce I/O and to improve performance.
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:

1. User information, such as user privileges
2. Integrity constraints defined for tables in the database
3. Names and datatypes of all columns in database tables
4. 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.


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):

1. Archiver processes (ARCn)
2. Checkpoint process (CKPT) *REQUIRED*
3. Coordinator-of-job-queues process (CJQn): dynamically spawns slave processes for job-queues
4. Database writer processes (DBWn) *REQUIRED*
5. Dispatcher processes (Dnnn): multiplex server-processes on behalf of users
6. Memory-manager process (MMAN): used for internal database tasks such as Automatic
7. Shared Memory Management
8. Log-writer process (LGWR) *REQUIRED*
9. Log-write network-server (LNSn): transmits redo logs in Data Guard environments
10. Logical standby coordinator process (LSP0): controls Data Guard log-application
11. Media-recovery process (MRP): detached recovery-server process
12. Memory-monitor process (MMON): process for automatic problem-detection, self-tuning and statistics-gathering
13. Memory-monitor light process (MMNL): gathers and stores Automatic Workload
14. Repository (AWR) data
15. Mmon slaves (Mnnnn—M0000, M0001, etc): background slaves of the MMON process
16. Process-monitor process (PMON) *REQUIRED*
17. Process-spawner (PSP0): spawns Oracle processes
18. Queue-monitor processes (QMNn)
19. Recoverer process (RECO)
20. Remote file-server process (RFS)
21. Shared server processes (Snnn): serve client-requests
22. 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).

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.

Version numbering
Oracle products have historically followed their own release-numbering and naming conventions. With the Oracle RDBMS 10g release, Oracle Corporation started standardizing all current versions of its major products using the "10g" label, although some sources continued to refer to Oracle Applications Release 11i as Oracle 11i. Major database-related products and some of their versions include:
Oracle Application Server 10g (also known as "Oracle AS 10g"): a middleware product;
Oracle Applications Release 11i (aka Oracle e-Business Suite, Oracle Financials or Oracle 11i): a suite of business applications;
Oracle Developer Suite 10g (9.0.4);
Oracle JDeveloper 10g: a Java integrated development environment;

Since version 5, Oracle's RDBMS release numbering has used the following codes:

1. Oracle5
2. Oracle6
3. Oracle7: 7.0.16—7.3.4
4. Oracle8 Database: 8.0.3—8.0.6
5. Oracle8i Database Release 1: 8.1.5.0—8.1.5.1
6. Oracle8i Database Release 2: 8.1.6.0—8.1.6.3
7. Oracle8i Database Release 3: 8.1.7.0—8.1.7.4
8. Oracle9i Database Release 1: 9.0.1.0—9.0.1.5 (patchset as of December 2003)
9. Oracle9i Database Release 2: 9.2.0.1—9.2.0.8 (patchset as of April 2007)
10. Oracle Database 10g Release 1: 10.1.0.2—10.1.0.5 (patchset as of February 2006)
11. Oracle Database 10g Release 2: 10.2.0.1—10.2.0.4 (patchset as of April 2008)
12. Oracle Database 11g Release 1: 11.1.0.6—11.1.0.7 (patchset as of September 2008)
13. Oracle Database 11g Release 2: 11.2.0.1 (released 2009-09-01)

The version-numbering syntax within each release follows the pattern: major.maintenance.application-server.component-specific.platform-specific.

For example, "10.2.0.1 for 64-bit Solaris" means: 10th major version of Oracle, maintenance level 2, Oracle Application Server (OracleAS) 0, level 1 for Solaris 64-bit.

The Oracle Administrator's Guide offers further information on Oracle release numbers. Oracle Corporation provides a table showing the latest patch-set releases by major release, operating-system, and hardware-architecture.