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Oracle9i Database Concepts
Release 2 (9.2)

Part Number A96524-01
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AFTER trigger

When defining a trigger, you can specify the trigger timing--whether the trigger action is to be executed before or after the triggering statement.

AFTER triggers execute the trigger action after the triggering statement is executed.

BEFORE and AFTER apply to both statement and row triggers.

See Also: trigger


See: Oracle architecture

archived redo log

Optionally, filled online redo files can be archived before being reused, creating an archived redo log. Archived (offline) redo log files constitute the archived redo log.

See Also: redo log

background process

Background processes consolidate functions that would otherwise be handled by multiple Oracle programs running for each user process. The background processes asynchronously perform I/O and monitor other Oracle processes to provide increased parallelism for better performance and reliability.

Oracle creates a set of background processes for each instance.

See Also: instance, process, Oracle process, user process

BEFORE trigger

When defining a trigger, you can specify the trigger timing--whether the trigger action is to be executed before or after the triggering statement.

BEFORE triggers execute the trigger action before the triggering statement is executed.

BEFORE and AFTER apply to both statement and row triggers.

See Also: trigger

buffer cache

The database buffer cache is the portion of the SGA that holds copies of data blocks read from data files. All user processes concurrently connected to the instance share access to the database buffer cache.

See Also: system global area (SGA)

byte semantics

The length of string is measured in bytes.

character semantics

The length of string is measured in characters.

CHECK constraint

A CHECK integrity constraint on a column or set of columns requires that a specified condition be true or unknown for every row of the table. If a DML statement results in the condition of the CHECK constraint evaluating to false, then the statement is rolled back.


In client/server architecture, the front-end database application, which interacts with a user through the keyboard, display, and pointing device such as a mouse. The client portion has no data access responsibilities. It concentrates on requesting, processing, and presenting data managed by the server portion.

See Also: client/server architecture, server

client/server architecture

Software architecture based on a separation of processing between two CPUs, one acting as the client in the transaction, requesting and receiving services, and the other as the server that provides services in a transaction.


Optional structure for storing table data. Clusters are groups of one or more tables physically stored together because they share common columns and are often used together. Because related rows are physically stored together, disk access time improves.


Vertical space in a database table that represents a particular domain of data. A column has a column name and a specific datatype. For example, in a table of employee information, all of the employees' dates of hire would constitute one column.

See Also: row, table


Make permanent changes to data (inserts, updates, deletes) in the database. Before changes are committed, both the old and new data exist so that changes can be stored or the data can be restored to its prior state.

See Also: roll back


Simultaneous access of the same data by many users. A multiuser database management system must provide adequate concurrency controls, so that data cannot be updated or changed improperly, compromising data integrity.

See Also: data consistency


Communication pathway between a user process and an Oracle instance.

See Also: session, user process

control file

A file that records the physical structure of a database and contains the database name, the names and locations of associated databases and online redo log files, the time stamp of the database creation, the current log sequence number, and checkpoint information.

See Also: physical structures, redo log


Collection of data that is treated as a unit. The purpose of a database is to store and retrieve related information.

database buffer

One of several types of memory structures that stores information within the system global area. Database buffers store the most recently used blocks of data.

See Also: system global area (SGA)

database buffer cache

Memory structure in the system global area that stores the most recently used blocks of data.

See Also: system global area (SGA)

database link

A named schema object that describes a path from one database to another. Database links are implicitly used when a reference is made to a global object name in a distributed database.

database writer process (DBWn)

An Oracle background process that writes the contents of buffers to data files. The DBWn processes are responsible for writing modified (dirty) buffers in the database buffer cache to disk.

See Also: buffer cache

data block

Smallest logical unit of data storage in an Oracle database. Also called logical blocks, Oracle blocks, or pages. One data block corresponds to a specific number of bytes of physical database space on disk.

See Also: extent, segment

data consistency

In a multiuser environment, where many users can access data at the same time (concurrency), data consistency means that each user sees a consistent view of the data, including visible changes made by the user's own transactions and transactions of other users.

See Also: concurrency

data dictionary

The central set of tables and views that are used as a read-only reference about a particular database. A data dictionary stores such information as:

A data dictionary is created when a database is created and is automatically updated when the structure of the database is updated.

data integrity

Business rules that dictate the standards for acceptable data. These rules are applied to a database by using integrity constraints and triggers to prevent the entry of invalid information into tables.

See Also: integrity constraint, trigger

data file

A physical operating system file on disk that was created by Oracle and contains data structures such as tables and indexes. A data file can only belong to one database.

See Also: index, physical structures

data segment

Each nonclustered table has a data segment. All of the table's data is stored in the extents of its data segment. For a partitioned table, each partition has a data segment.

Each cluster has a data segment. The data of every table in the cluster is stored in the cluster's data segment.

See Also: cluster, extent, segment


Database time zone.

dedicated server

A database server configuration in which a server process handles requests for a single user process.

See Also: shared server

dispatcher processes (Dnnn)

Optional background processes, present only when a shared server configuration is used. At least one dispatcher process is created for every communication protocol in use (D000, . . ., Dnnn). Each dispatcher process is responsible for routing requests from connected user processes to available shared server processes and returning the responses back to the appropriate user processes.

See Also: shared server

distributed processing

Software architecture that uses more than one computer to divide the processing for a set of related jobs. Distributed processing reduces the processing load on a single computer.


Data definition language. Includes statements like CREATE/ALTER TABLE/INDEX, which define or change data structure.


Data manipulation language. Includes statements like INSERT, UPDATE, and DELETE, which change data in tables.


The degree of parallelism of an operation.

Enterprise Manager

An Oracle system management tool that provides an integrated solution for centrally managing your heterogeneous environment. It combines a graphical console, Oracle Management Servers, Oracle Intelligent Agents, common services, and administrative tools for managing Oracle products.


Second level of logical database storage. An extent is a specific number of contiguous data blocks allocated for storing a specific type of information.

See Also: data block, segment

flashback query

Uses Oracle's multiversion read-consistency capabilities to restore data by applying undo as needed. You can view and repair historical data, and you can perform queries on the database as of a certain wall clock time or user-specified system commit number (SCN).

foreign key

Integrity constraint that requires each value in a column or set of columns to match a value in a related table's UNIQUE or PRIMARY KEY.

FOREIGN KEY integrity constraints also define referential integrity actions that dictate what Oracle should do with dependent data if the data it references is altered.

See Also: integrity constraint, primary key


Optional structure associated with tables and clusters. You can create indexes on one or more columns of a table to speed SQL statement execution on that table.

See Also: cluster

index segment

Each index has an index segment that stores all of its data. For a partitioned index, each partition has an index segment.

See Also: index, segment


An object that registers a new indexing scheme by specifying the set of supported operators and routines that manage a domain index.


A system global area (SGA) and the Oracle background processes constitute an Oracle instance. Every time a database is started, a system global area is allocated and Oracle background processes are started. The SGA is deallocated when the instance shuts down.

See Also: background process, system global area (SGA)


See: data integrity

integrity constraint

Declarative method of defining a rule for a column of a table. Integrity constraints enforce the business rules associated with a database and prevent the entry of invalid information into tables.


Column or set of columns included in the definition of certain types of integrity constraints. Keys describe the relationships between the different tables and columns of a relational database.

See Also: integrity constraint, foreign key, primary key

large pool

Optional area in the system global area that provides large memory allocations for Oracle backup and restore operations, I/O server processes, and session memory for the shared server and Oracle XA.

See Also: system global area (SGA), process, shared server, Oracle XA


A tool that lets administrators use SQL to read, analyze, and interpret log files. It can view any redo log file, online or archived. The Oracle Enterprise Manager application Oracle9i LogMiner Viewer adds a GUI-based interface.

log writer process (LGWR)

The log writer process (LGWR) is responsible for redo log buffer management--writing the redo log buffer to a redo log file on disk. LGWR writes all redo entries that have been copied into the buffer since the last time it wrote.

See Also: redo log

logical structures

Logical structures of an Oracle database include tablespaces, schema objects, data blocks, extents, and segments. Because the physical and logical structures are separate, the physical storage of data can be managed without affecting the access to logical storage structures.

See Also: physical structures

materialized view

A materialized view provides indirect access to table data by storing the results of a query in a separate schema object.

See Also: view

NOT NULL constraint

Data integrity constraint that requires a column of a table contain no null values.

See Also: NULL value

NULL value

Absence of a value in a column of a row. Nulls indicate missing, unknown, or inapplicable data. A null should not be used to imply any other value, such as zero.

object type

An object type consists of two parts: a spec and a body. The type body always depends on its type spec.

offline redo log

See: archived redo log

online redo log

The online redo log is a set of two or more online redo log files that record all changes made to the database, including both uncommitted and committed changes. Redo entries are temporarily stored in redo log buffers of the system global area, and the background process LGWR writes the redo entries sequentially to an online redo log file.

See Also: redo log, system global area (SGA), background process, log writer process (LGWR)


In memory management, the term operator refers to a data flow operator, such as a sort, hash join, or bitmap merge.

Oracle architecture

Memory and process structures used by an Oracle server to manage a database.

See Also: database, process, server

Oracle process

Oracle processes run the Oracle server code. They include server processes and background processes.

See Also: process, server process, background process, user process

Oracle XA

The Oracle XA library is an external interface that allows global transactions to be coordinated by a transaction manager other than the Oracle server.


A smaller and more manageable piece of a table or index.

physical structures

Physical database structures of an Oracle database include data files, redo log files, and control files.

See Also: logical structures


Oracle's procedural language extension to SQL. PL/SQL enables you to mix SQL statements with procedural constructs. With PL/SQL, you can define and execute PL/SQL program units such as procedures, functions, and packages.

See Also: SQL

primary key

The column or set of columns included in the definition of a table's PRIMARY KEY constraint. A primary key's values uniquely identify the rows in a table. Only one primary key can be defined for each table.

See Also: PRIMARY KEY constraint

PRIMARY KEY constraint

Integrity constraint that disallows duplicate values and nulls in a column or set of columns.

See Also: integrity constraint

priority inversion

Priority inversion occurs when a high priority job is executed with lower amount of resources than a low priority job. Thus the expected priority is "inverted."


Each process in an Oracle instance performs a specific job. By dividing the work of Oracle and database applications into several processes, multiple users and applications can connect to a single database instance simultaneously.

See Also: Oracle process, user process

program global area (PGA)

A memory buffer that contains data and control information for a server process. A PGA is created by Oracle when a server process is started. The information in a PGA depends on the Oracle configuration.

query block

A self-contained DML against a table. A query block can be a top-level DML or a subquery.

See Also: DML

read consistency

In a multiuser environment, Oracle's read consistency ensures that

See Also: concurrency, data consistency

Real Application Clusters

Option with Oracle9i Enterprise Edition that allows multiple concurrent instances to share a single physical database.

See Also: instance

Recovery Manager (RMAN)

An Oracle utility that manages backup and recovery operations, creates backups of database files (datafiles, control files, and archived redo log files), and restores or recovers a database from backups.

redo log

A set of files that protect altered database data in memory that has not been written to the datafiles. The redo log can consist of two parts: the online redo log and the archived redo log.

See Also: online redo log, archived redo log

redo log buffer

Memory structure in the system global area that stores redo entries--a log of changes made to the database. The redo entries stored in the redo log buffers are written to an online redo log file, which is used if database recovery is necessary.

See Also: system global area (SGA)

referential integrity

A rule defined on a key (a column or set of columns) in one table that guarantees that the values in that key match the values in a key in a related table (the referenced value).

Referential integrity also includes the rules that dictate what types of data manipulation are allowed on referenced values and how these actions affect dependent values.

See Also: key

roll back

Undo any changes to data that have been performed by SQL statements within an uncommitted transaction. After a transaction has been committed, it cannot be rolled back.

Oracle uses rollback segments to store old values. The redo log contains a record of changes.

See Also: commit, transaction, rollback segment

rollback segment

Logical database structure created by the database administrator to temporarily store undo information. Rollback segments store old data changed by SQL statements in a transaction until it is committed.

See Also: commit, logical structures, segment


Set of attributes or values pertaining to one entity or record in a table. A row is a collection of column information corresponding to a single record.

See Also: column, table


A globally unique identifier for a row in a database. It is created at the time the row is inserted into a table, and destroyed when it is removed from a table.


Collection of database objects, including logical structures such as tables, views, sequences, stored procedures, synonyms, indexes, clusters, and database links. A schema has the name of the user who controls it.

See Also: logical structures


Current session time zone.


Third level of logical database storage. A segment is a set of extents, each of which has been allocated for a specific data structure, and all of which are stored in the same tablespace.

See Also: extent, data block


A sequence generates a serial list of unique numbers for numeric columns of a database's tables.


In a client/server architecture, the computer that runs Oracle software and handles the functions required for concurrent, shared data access. The server receives and processes the SQL and PL/SQL statements that originate from client applications.

See Also: client, client/server architecture

server process

Server processes handle requests from connected user processes. A server process is in charge of communicating with the user process and interacting with Oracle to carry out requests of the associated user process.

See Also: process, user process


Specific connection of a user to an Oracle instance through a user process. A session lasts from the time the user connects until the time the user disconnects or exits the database application.

See Also: connection, instance, user process

shared pool

Portion of the system global area that contains shared memory constructs such as shared SQL areas. A shared SQL area is required to process every unique SQL statement submitted to a database.

See Also: system global area (SGA), SQL

shared server

A database server configuration that allows many user processes to share a small number of server processes, minimizing the number of server processes and maximizing the use of available system resources.

See Also: dedicated server


Structured Query Language, a nonprocedural language to access data. Users describe in SQL what they want done, and the SQL language compiler automatically generates a procedure to navigate the database and perform the desired task.

See Also: SQL*Plus, PL/SQL


Oracle tool used to execute SQL statements against an Oracle database. Oracle SQL includes many extensions to the ANSI/ISO standard SQL language.

See Also: SQL, PL/SQL


In the hierarchy of user-defined datatypes, a subtype is always a dependent on its supertype.


See: subtype


An alias for a table, view, sequence, or program unit.

system global area (SGA)

A group of shared memory structures that contain data and control information for one Oracle database instance. If multiple users are concurrently connected to the same instance, then the data in the instance's SGA is shared among the users. Consequently, the SGA is sometimes referred to as the shared global area.

See Also: instance


Basic unit of data storage in an Oracle database. Table data is stored in rows and columns.

See Also: column, row


A database storage unit that groups related logical structures together.

See Also: logical structures

temporary segment

Temporary segments are created by Oracle when a SQL statement needs a temporary work area to complete execution. When the statement finishes execution, the temporary segment's extents are returned to the system for future use.

See Also: extent, segment


Logical unit of work that contains one or more SQL statements. All statements in a transaction are committed or rolled back together.

See Also: commit, roll back


Stored database procedure automatically invoked whenever a table or view is modified, for example by INSERT, UPDATE, or DELETE operations.


A way of representing all the characters in all the languages in the world. Characters are defined as a sequence of codepoints, a base codepoint followed by any number of surrogates. There are 64K codepoints.

Unicode column

A column of type NCHAR, NVARCHAR2, or NCLOB in Oracle9i or later. It is guaranteed to be able to hold unicode.

UNIQUE key constraint

A data integrity constraint requiring that every value in a column or set of columns (key) be unique--that is, no two rows of a table have duplicate values in a specified column or set of columns.

See Also: integrity constraint, key

user name

The name by which a user is known to the Oracle server and to other users. Every user name is associated with a password, and both must be entered to connect to an Oracle database.

user process

User processes execute the application or Oracle tool code.

See Also: process, Oracle process


Coordinated Universal Time, previously called Greenwich Mean Time, or GMT.


A view is a custom-tailored presentation of the data in one or more tables. A view can also be thought of as a "stored query." Views do not actually contain or store data; they derive their data from the tables on which they are based.

Like tables, views can be queried, updated, inserted into, and deleted from, with some restrictions. All operations performed on a view affect the base tables of the view.

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