Что такое oracle database
Национальная библиотека им. Н. Э. Баумана
Bauman National Library
Персональные инструменты
Oracle Database
Oracle Database или Oracle RDBMS — объектно-реляционная система управления базами данных компании Oracle. [Источник 1]
Содержание
История
История выпуска версий для различных операционных платформ
История выпуска для Linux x86
сентябрь 1998 года — 8.0 (8.0.5) 23 февраля 1999 года — 8.0 (8.0.5.1.0) 22 ноября 2000 года — 8i Release 3 (8.1.7.0.1) 25 марта 2003 года — 9i Release 2 (9.2.0.4) 21 декабря 2004 года — 10g Release 1 (10.1.0.3) 2 июля 2005 года — 10g Release 2 (10.2.0.1) 10 августа 2007 года — 11g Release 1 (11.1.0.6) 1 сентября 2009 года — 11g Release 2 (11.2.0.1)
История выпуска для Linux x86-64
16 октября 2007 года — 11g Release 1 (11.1.0.6) 1 сентября 2009 года — 11g Release 2 (11.2.0.1) 26 июня 2013 года — 12c (12.1.0.1)
История выпуска для Solaris x86
14 мая 1999 года — 8i Release 1 (8.1.5) для Intel UNIX (DG/UX Intel, SCO UnixWare, Solaris Intel)
История выпуска для Solaris x86-64
23 марта 2006 года — 10g Release 2 (10.2.0.1) 25 ноября 2009 года — 11g Release 2 (11.2.0.1) 26 июня 2013 года — 12c (12.1.0.1)
История выпуска для Solaris SPARC 64-bit
6 ноября 2009 года — 11g Release 2 (11.2.0.1) 26 июня 2013 года — 12c (12.1.0.1)
История выпуска для Windows x86
март 1997 года — 7 (7.3.3) для Windows NT 3.51/4.0 октябрь 1997 года — 7 (7.3.4) для Windows NT 3.51/4.0 1 июля 1998 года — 8.0 (8.0.5) для Windows NT 10 марта 1999 года — 8i Release 1 (8.1.5) для Windows NT и Windows 95/98 20 сентября 1999 года — 8.0 (8.0.6) для Windows NT январь 2000 года — 8i Release 2 (8.1.6) для Windows NT 16 ноября 2000 года — 8i Release 3 (8.1.7) для Windows NT 13 сентября 2001 года — 9i Release 1 (9.0.1.0) для Windows 32-bit 14 мая 2002 года — 9i Release 2 (9.2.0.1) для Windows 32-bit 26 марта 2004 года — 10g Release 1 (10.1.0.2) для Windows 32-bit 7 сентября 2005 года — 10g Release 2 (10.2.0.1) для Windows 32-bit 15 октября 2007 года — 11g Release 1 (11.1.0.6) для Windows 32-bit 5 апреля 2010 года — 11g Release 2 (11.2.0.1) для Windows 32-bit
История выпуска для Windows x86-64
31 октября 2005 года — 10g Release 2 (10.2.0.1) 7 ноября 2007 года — 11g Release 1 (11.1.0.6) 2 апреля 2010 года — 11g Release 2 (11.2.0.1) 1 августа 2013 года — 12c Release 1 (12.1.0.1) 8 марта 2017 года — 12c Release 2 (12.2.0.1)
Программно-аппаратные платформы
До выпуска Oracle9i корпорация Oracle портировала движок базы данных на многие платформы, но в последнее время Oracle портирует на меньшее количество платформ. К примеру Oracle RDBMS 10g с июня 2005 года поддерживаются следующие программно-аппаратные платформы:
Редакции
СУБД поставляется в шести различных редакциях, ориентированных на различные сценарии разработки и развертывания приложений (а также отличающиеся ценой).
| Название | Ограничения | Операционные платформы |
|---|---|---|
| Enterprise Edition | ||
| Standard Edition | не может устанавливаться на системы, имеющие более 4 процессорных разъёмов | |
| Standard Edition One | не может устанавливаться на системы, имеющие более 2 процессорных разъёмов; не поддерживает кластеризацию (RAC) | |
| Personal Edition | один пользователь | |
| Lite | для мобильных и встраиваемых устройств | |
| Express Edition (XE) | бесплатная редакция; используемая оперативная память — 1 ГБ, а также используется только 1 процессор, максимальный объём базы данных — 11 ГБ (для 10g — 4ГБ), из них от 0,5 до 0,9 ГБ используются словарём данных, внутренними схемами и временным дисковым пространством | Windows x86-64 Linux x86-64 |
Особенности
Пример работы с БД
Создание представления базы данных в Oracle с помощью SQL
Уровень сложности: Начальный
Требования к данным: Используйте собственные данные
Для отображения таблиц и классов пространственных объектов многопользовательской базы геоданных можно использовать SQL.
Приведенные в настоящей теме примеры показывают, как создать в Oracle простое представление для просмотра с ограничением доступа пользователей к другим столбцам. Этот пример построен на базе таблицы со следующим определением:
Предоставление прав доступа к таблице
Если создающий представление пользователь не является владельцем таблицы или таблиц в этом представлении, владелец таблиц должен предоставить создателю как минимум права доступа Select. Для предоставления прав доступа к представлению для других пользователей владелец представления должен получить соответствующие права доступа от владельца таблицы.
В данном примере таблица, на базе которой построено представление (employees), принадлежит пользователю gdb. Представление создается пользователем rocket. Также пользователь rocket предоставляет права доступа к представлению другим пользователям. Таким образом, пользователь gdb должен передать пользователю rocket права доступа SELECT и WITH GRANT OPTION, чтобы пользователь rocket мог передавать другим пользователям права доступа SELECT.
Создание представления
В этом примере пользователь rocket создает представление таблицы employees и ограничивает доступ к нему только пользователям из отдела 201:
Выдача прав доступа к представлению
Права доступа к представлению можно предоставлять определенным пользователям, не передавая им права доступа к базовой таблице (employees). В данном примере пользователю mgr200 предоставлены права доступа SELECT к представлению view_dept_201:
Тестовые права доступа
Войдите в систему как mgr200 и выберите записи view_dept_201:
Как ожидалось, выводятся только записи для сотрудников отдела 201.
Основные команды
Войдите в систему SQL * Plus
Список разделов справки, доступных в SQL * Plus
Выполнять команды хоста
Показать системные переменные SQL * Plus или настройки среды
Изменение системных переменных SQL * Plus или настроек среды
Запуск базы данных
Подключение к базе данных
Редактировать содержимое SQL-буфера или файла
Выполнять команды, хранящиеся в буфере SQL
Отключиться от базы данных
Проверить статус листенера
Зайти под администратором
Стартовать БД на standby
Посмотреть список датафайлов в табличном пространстве
Добавить к табличному пространству файл данных размером
Посмотреть список пользователей
Запустить накат архивных логов на standby
Остановить накат архивных логов на standby
Посмотреть на standby, какие архивные логи зарегистрированы и какие из них накатились
Зарегистрировать архивный лог на standby
Установка
Установка Oracle Database 12c на Windows 10 Professional 64 bit: [Источник 2]
1 Introduction to the Oracle Database
This chapter provides an overview of the Oracle database server. The topics include:
Oracle Database Architecture
An Oracle database is a collection of data treated as a unit. The purpose of a database is to store and retrieve related information. A database server is the key to solving the problems of information management. In general, a server reliably manages a large amount of data in a multiuser environment so that many users can concurrently access the same data. All this is accomplished while delivering high performance. A database server also prevents unauthorized access and provides efficient solutions for failure recovery.
Oracle Database is the first database designed for enterprise grid computing, the most flexible and cost effective way to manage information and applications. Enterprise grid computing creates large pools of industry-standard, modular storage and servers. With this architecture, each new system can be rapidly provisioned from the pool of components. There is no need for peak workloads, because capacity can be easily added or reallocated from the resource pools as needed.
The section contains the following topics:
Overview of Oracle Grid Architecture
Grid computing is a new IT architecture that produces more resilient and lower cost enterprise information systems. With grid computing, groups of independent, modular hardware and software components can be connected and rejoined on demand to meet the changing needs of businesses.
The grid style of computing aims to solve some common problems with enterprise IT: the problem of application silos that lead to under utilized, dedicated hardware resources, the problem of monolithic, unwieldy systems that are expensive to maintain and difficult to change, and the problem of fragmented and disintegrated information that cannot be fully exploited by the enterprise as a whole.
Benefits of Grid Computing Compared to other models of computing, IT systems designed and implemented in the grid style deliver higher quality of service, lower cost, and greater flexibility. Higher quality of service results from having no single point of failure, a robust security infrastructure, and centralized, policy-driven management. Lower costs derive from increasing the utilization of resources and dramatically reducing management and maintenance costs. Rather than dedicating a stack of software and hardware to a specific task, all resources are pooled and allocated on demand, thus eliminating under utilized capacity and redundant capabilities. Grid computing also enables the use of smaller individual hardware components, thus reducing the cost of each individual component and providing more flexibility to devote resources in accordance with changing needs.
Grid Computing Defined
The grid style of computing treats collections of similar IT resources holistically as a single pool, while exploiting the distinct nature of individual resources within the pool. To address simultaneously the problems of monolithic systems and fragmented resources, grid computing achieves a balance between the benefits of holistic resource management and flexible independent resource control. IT resources managed in a grid include:
Infrastructure: the hardware and software that create a data storage and program execution environment
Applications: the program logic and flow that define specific business processes
Information: the meanings inherent in all different types of data used to conduct business
Core Tenets of Grid Computing Two core tenets uniquely distinguish grid computing from other styles of computing, such as mainframe, client-server, or multi-tier: virtualization and provisioning.
With virtualization, individual resources (e.g. computers, disks, application components and information sources) are pooled together by type then made available to consumers (e.g. people or software programs) through an abstraction. Virtualization means breaking hard-coded connections between providers and consumers of resources, and preparing a resource to serve a particular need without the consumer caring how that is accomplished.
With provisioning, when consumers request resources through a virtualization layer, behind the scenes a specific resource is identified to fulfill the request and then it is allocated to the consumer. Provisioning as part of grid computing means that the system determines how to meet the specific need of the consumer, while optimizing operation of the system as a whole.
The specific ways in which information, application or infrastructure resources are virtualized and provisioned are specific to the type of resource, but the concepts apply universally. Similarly, the specific benefits derived from grid computing are particular to each type of resource, but all share the characteristics of better quality, lower costs and increased flexibility.
Infrastructure Grid Infrastructure grid resources include hardware resources such as storage, processors, memory, and networks as well as software designed to manage this hardware, such as databases, storage management, system management, application servers, and operating systems.
Virtualization and provisioning of infrastructure resources mean pooling resources together and allocating to the appropriate consumers based on policies. For example, one policy might be to dedicate enough processing power to a web server that it can always provide sub-second response time. That rule could be fulfilled in different ways by the provisioning software in order to balance the requests of all consumers.
Treating infrastructure resources as a single pool and allocating those resources on demand saves money by eliminating under utilized capacity and redundant capabilities. Managing hardware and software resources holistically reduces the cost of labor and the opportunity for human error.
Spreading computing capacity among many different computers and spreading storage capacity across multiple disks and disk groups removes single points of failure so that if any individual component fails, the system as a whole remains available. Furthermore, grid computing affords the option to use smaller individual hardware components, such as blade servers and low cost storage, which enables incremental scaling and reduces the cost of each individual component, thereby giving companies more flexibility and lower cost.
Infrastructure is the dimension of grid computing that is most familiar and easy to understand, but the same concepts apply to applications and information.
Applications Grid Application resources in the grid are the encodings of business logic and process flow within application software. These may be packaged applications or custom applications, written in any programming language, reflecting any level of complexity. For example, the software that takes an order from a customer and sends an acknowledgement, the process that prints payroll checks, and the logic that routes a particular customer call to a particular agent are all application resources.
Historically, application logic has been intertwined with user interface code, data management code, and process or page flow and has lacked well-defined interfaces, which has resulted in monolithic applications that are difficult to change and difficult to integrate.
Service oriented architecture has emerged as a superior model for building applications, and service oriented architecture concepts align exactly with the core tenets of grid computing. Virtualization and provisioning of application resources involves publishing application components as services for use by multiple consumers, which may be people or processes, then orchestrating those services into more powerful business flows.
In the same way that grid computing enables better reuse and more flexibility of IT infrastructure resources, grid computing also treats bits of application logic as a resource, and enables greater reuse of application functionality and more flexibility in changing and building new composite applications.
Furthermore, applications that are orchestrated from published services are able to view activities in a business as a single whole, so that processes are standardized across geography and business units and processes are automated end-to-end. This generates more reliable business processes and lowers cost through increased automation and reduced variability.
Information Grid The third dimension to grid computing, after infrastructure and applications, is information. Today, information tends to be fragmented across a company, making it difficult to see the business as a whole or answer basic questions.about customers. Without information about who the customer is, and what they want to buy, information assets go underexploited.
In contrast, grid computing treats information holistically as a resource, similar to infrastructure and applications resources, and thus extracts more of its latent value. Information grid resources include all data in the enterprise and all metadata required to make that data meaningful. This data may be structured, semi-structured, or unstructured, stored in any location, such as databases, local file systems, or e-mail servers, and created by any application.
The core tenets of grid computing apply similarly to information as they do to infrastructure and applications. The infrastructure grid exploits the power of the network to allow multiple servers or storage devices to be combined toward a single task, then easily reconfigured as needs change. A service oriented architecture, or an applications grid, enables independently developed services, or application resources, to be combined into larger business processes, then adapted as needs change without breaking other parts of the composite application. Similarly, the information grid provides a way for information resources to be joined with related information resources to greater exploit the value of the inherent relationships among information, then for new connections to be made as situations change.
The relational database, for example, was an early information virtualization technology. Unlike its predecessors, the network database and hierarchical database models, in which all relationships between data had to be predetermined, relational database enabled flexible access to a general-purpose information resource. Today, XML furthers information virtualization by providing a standard way to represent information along with metadata, which breaks the hard link between information and a specific application used to create and view that information.
Information provisioning technologies include message queuing, data propagation, replication, extract-transform-load, as well as mapping and cleansing tools to ensure data quality. Data hubs, in which a central operational data store continually syncs with multiple live data sources, are emerging as a preferred model for establishing a single source of truth while maintaining the flexibility of distributed control.
Grid Computing in Oracle Database 10 g
On the path toward this grand vision of grid computing, companies need real solutions to support their incremental moves toward a more flexible and more productive IT architecture. The Oracle Database 10 g family of software products implements much of the core grid technology to get companies started. And Oracle delivers this grid computing functionality in the context of holistic enterprise architecture, providing a robust security infrastructure, centralized management, intuitive, powerful development tools, and universal access. Oracle Database 10g includes:
Oracle Database 10 g
Oracle Application Server 10 g
Oracle Enterprise Manager 10 g
Oracle Collaboration Suite 10 g
Although the grid features of Oracle 10g span all of the products listed above, this discussion will focus on the grid computing capabilities of Oracle Database 10g.
Server Virtualization. Oracle Real Application Clusters 10 g (RAC) enable a single database to run across multiple clustered nodes in a grid, pooling the processing resources of several standard machines. Oracle is uniquely flexible in its ability to provision workload across machines because it is the only database technology that does not require data to be partitioned and distributed along with the work. Oracle 10 g Release 2 software includes enhancements for balancing connections across RAC instances, based on policies.
Storage Virtualization. The Oracle Automatic Storage Management (ASM) feature of Oracle Database 10 g provides a virtualization layer between the database and storage so that multiple disks can be treated as a single disk group and disks can be dynamically added or removed while keeping databases online. Existing data will automatically be spread across available disks for performance and utilization optimization. In Oracle 10 g Release 2, ASM supports multiple databases, which could be at different software version levels, accessing the same storage pool.
Grid Management. Because grid computing pools together multiple servers and disks and allocates them to multiple purposes, it becomes more important that individual resources are largely self-managing and that other management functions are centralized.
The Grid Control feature of Oracle Enterprise Manager 10 g provides a single console to manage multiple systems together as a logical group. Grid Control manages provisioning of nodes in the grid with the appropriate full stack of software and enables configurations and security settings to be maintained centrally for groups of systems.
Oracle Enterprise Manager 10 g enhances Oracle’s support for service oriented architectures by monitoring and managing web services and any other administrator-defined services, tracking end-to-end performance and performing root cause analysis of problems encountered.
Data Provisioning. Information starts with data, which must be provisioned wherever consumers need it. For example, users may be geographically distributed, and fast data access may be more important for these users than access to an identical resource. In these cases, data must be shared between systems, either in bulk or near real time. Oracle’s bulk data movement technologies include Transportable Tablespaces and Data Pump.
For more fine-grained data sharing, the Oracle Streams feature of Oracle Database 10 g captures database transaction changes and propagates them, thus keeping two or more database copies in sync as updates are applied. It also unifies traditionally distinct data sharing mechanisms, such as message queuing, replication, events, data warehouse loading, notifications and publish/subscribe, into a single technology.
Centralized Data Management. Oracle Database 10 g manages all types of structured, semi-structured and unstructured information, representing, maintaining and querying each in its own optimal way while providing common access to all via SQL and XML Query. Along with traditional relational database structures, Oracle natively implements OLAP cubes, standard XML structures, geographic spatial data and unlimited sized file management, thus virtualizing information representation. Combining these information types enables connections between disparate types of information to be made as readily as new connections are made with traditional relational data.
Metadata Management. Oracle Warehouse Builder is more than a traditional batch ETL tool for creating warehouses. It enforces rules to achieve data quality, does fuzzy matching to automatically overcome data inconsistency, and uses statistical analysis to infer data profiles. With Oracle 10 g Release 2, its metadata management capabilities are extended from scheduled data pulls to handle a transaction-time data push from an Oracle database implementing the Oracle Streams feature.
Oracle’s series of enterprise data hub products (for example, Oracle Customer Data Hub) provide real-time synchronization of operational information sources so that companies can have a single source of truth while retaining separate systems and separate applications, which may include a combination of packaged, legacy and custom applications. In addition to the data cleansing and scheduling mechanisms, Oracle also provides a well-formed schema, established from years of experience building enterprise applications, for certain common types of information, such as customer, financial, and product information.
Metadata Inference. Joining the Oracle 10 g software family is the new Oracle Enterprise Search product. Oracle Enterprise Search 10 g crawls all information sources in the enterprise, whether public or secure, including e-mail servers, document management servers, file systems, web sites, databases and applications, then returns information from all of the most relevant sources for a given search query. This crawl and index process uses a series of heuristics specific to each data source to infer metadata about all enterprise information that is used to return the most relevant results to any query.
Overview of Application Architecture
There are two common ways to architect a database: client/server or multitier. As internet computing becomes more prevalent in computing environments, many database management systems are moving to a multitier environment.
Client/Server Architecture
Multiprocessing uses mo re than one processor for a set of related jobs. Distributed processing reduces the load on a single processor by allowing different processors to concentrate on a subset of related tasks, thus improving the performance and capabilities of the system as a whole.
The Client
The client is a database application that initiates a request for an operation to be performed on the database server. It requests, processes, and presents data managed by the server. The client workstation can be optimized for its job. For example, it might not need large disk capacity, or it might benefit from graphic capabilities.
Often, the client runs on a different computer than the database server, generally on a PC. Many clients can simultaneously run against one server.
The Server
The server 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. The computer that manages the server can be optimized for its duties. For example, it can have large disk capacity and fast processors.
Multitier Architecture: Application Servers
A multitier architecture has the following components:
A client or initiator process that starts an operation
One or more application servers that perform parts of the operation. An application server provides access to the data for the client and performs some of the query processing, thus removing some of the load from the database server. It can serve as an interface between clients and multiple database servers, including providing an additional level of security.
An end or database server that stores most of the data used in the operation
This architecture enables use of an application server to do the following:
Validate the credentials of a client, such as a Web browser
Connect to an Oracle database server
Perform the requested operation on behalf of the client
If proxy authentication is being used, then the identity of the client is maintained throughout all tiers of the connection.
Overview of Physical Database Structures
The following sections explain the physical database structures of an Oracle database, including datafiles, redo log files, and control files.
Datafiles
The characteristics of datafiles are:
A datafile can be associated with only one database.
Datafiles can have certain characteristics set to let them automatically extend when the database runs out of space.
One or more datafiles form a logical unit of database storage called a tablespace.
Data in a datafile is read, as needed, during normal database operation and stored in the memory cache of Oracle. For example, assume that a user wants to access some data in a table of a database. 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, as determined by the database writer process (DBW n ) background process.
«Overview of the Oracle Instance» for more information about Oracle’s memory and process structures
Control Files
Names and locations of datafiles and redo log files
Time stamp of database creation
Oracle can multiplex the control file, that is, simultaneously maintain a number of identical control file copies, to protect against a failure involving the control file.
Every time an instance of an Oracle database is started, its control file identifies the database and redo log files that must be opened for database operation to proceed. If the physical makeup of the database is altered (for example, if a new datafile or redo log file is created), then the control file is automatically modified by Oracle to reflect the change. A control file is also used in database recovery.
Redo Log Files
The primary function of the redo log is to record all changes made to data. If a failure prevents modified data from being permanently written to the datafiles, then the changes can be obtained from the redo log, so work is never lost.
To protect against a failure involving the redo log itself, Oracle allows a multiplexed redo log so that two or more copies of the redo log can be maintained on different disks.
The information in a redo log file is used only to recover the database from a system or media failure that prevents database data from being written to the datafiles. For example, if an unexpected power outage terminates database operation, then data in memory cannot be written to the datafiles, and the data is lost. However, lost data can be recovered when the database is opened, after power is restored. By applying the information in the most recent redo log files to the database datafiles, Oracle restores the database to the time at which the power failure occurred.
Archive Log Files
You can en able automatic archiving of the redo log. Oracle automatically archives log files when the database is in ARCHIVELOG mode.
Parameter Files
Param eter files contain a list of configuration parameters for that instance and database.
Oracle recommends that you create a server parameter file (SPFILE) as a dynamic means of maintaining initialization parameters. A server parameter file lets you store and manage your initialization parameters persistently in a server-side disk file.
Oracle Database Administrator’s Guide for information on creating and changing parameter files
Alert and Trace Log Files
Each server and background process can write to an associated tra ce file. When an internal error is detected by a process, it dumps information about the error to its trace file. Some of the information written to a trace file is intended for the database administrator, while other information is for Oracle Support Services. Trace file information is also used to tune applications and instances.
The alert fi le, or alert log, is a special trace file. The alert log of a database is a chronological log of messages and errors.
Backup Files
To restore a file is to replace it with a backup file. Typically, you restore a file when a media failure or user error has damaged or deleted the original file.
User-managed backup and recovery requires you to actually restore backup files before you can perform a trial recovery of the backups.
Server-managed backup and recovery manages the backup process, such as scheduling of backups, as well as the recovery process, such as applying the correct backup file when recovery is needed.
Overview of Logical Database Structures
The logical storage structures, including data blocks, extents, and segments, enable Oracle to have fine-grained control of disk space use.
Tablespaces
Each database is logically divided into one or more tablespaces. One or more datafiles are explicitly created for each tablespace to physically store the data of all logical structures in a tablespace. The combined size of the datafiles in a tablespace is the total storage capacity of the tablespace.
Online and Offline Tablespaces
A tablespace can be online (accessible) or offline (not accessible). A tablespace is generally online, so that users can access the information in the tablespace. However, sometimes a tablespace is taken offline to make a portion of the database unavailable while allowing normal access to the remainder of the database. This makes many administrative tasks easier to perform.
Oracle Data Blocks
Extents
Segments
| Segment | Description |
|---|---|
| D ata segment | Each nonclustered table has a data segment. All table data is stored in the extents of the 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.
For a partitioned index, each partition has an index segment.
Earlier releases of Oracle used rollback segments to store undo information. The information in a rollback segment was used during database recovery for generating read-consistent database information and for rolling back uncommitted transaction s for users.
Space management for these rollback segments was complex, and Oracle has deprecated that method. This book discusses the undo tablespace method of managing undo; this eliminates the complexities of managing rollback segment space, and lets you exert control over how long undo is retained before being overwritten.
Oracle does use a SYSTEM rollback segment for performing system transactions. There is only one SYSTEM rollback segment and it is created automatically at CREATE DATABASE time and is always brought online at instance startup. You are not required to perform any operations to manage the SYSTEM rollback segment.
Oracle dynamically allocates space when the existing extents of a segment become full. In other words, when the extents of a segment are full, Oracle allocates another extent for that segment. Because extents are allocated as needed, the extents of a segment may or may not be contiguous on disk.