| Oracle® Database Concepts 11g Release 2 (11.2) Part Number E10713-02 |
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| Oracle® Database Concepts 11g Release 2 (11.2) Part Number E10713-02 |
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View PDF |
This chapter provides an overview of Oracle Database.
This chapter contains the following sections:
Every organization has information that it must store and manage to meet its requirements. For example, a corporation must collect and maintain human resources records for its employees. This information must be available to those who need it. An information system is a formal system for storing and processing information.
An information system could be a set of cardboard boxes containing files along with rules for how to store and retrieve the files. However, most companies today use a database to automate their information systems. A database is an organized collection of information treated as a unit. The purpose of a database is to collect, store, and retrieve related information for use by database applications.
A database management system (DBMS) is software that controls the storage, organization, and retrieval of data. Typically, a DBMS has the following elements:
Kernel code
This code manages memory and storage for the DBMS.
Repository of metadata
This repository is usually called a data dictionary.
Query language
This language enables users to access the data.
A database application is a software program that interacts with a database to access and manipulate data.
The first generation of database management systems included the following types:
Hierarchical
A hierarchical database organizes data in a tree structure. Each parent record has one or more child records, similar to the structure of a file system.
Network
A network database is similar to a hierarchical database, except records have a many-to-many rather than a one-to-many relationship.
The preceding database management systems stored data in rigid, predetermined relationships. Because no data definition language existed, changing the structure of the data was difficult. Also, these systems lacked a simple query language, which hindered application development.
In his seminal 1970 paper "A Relational Model of Data for Large Shared Data Banks," E. F. Codd defined a relational model based on mathematical set theory. Today, the most widely accepted database model is the relational model.
A relational database is a database that conforms to the relational model. The relational model has the following major aspects:
Structures
Well-defined objects store or access the data of a database.
Operations
Clearly defined actions enable users to manipulate the data and structures of a database.
Integrity rules
Integrity rules govern operations on the data and structures of a database.
A relational database stores data in a set of simple relations. A relation is a set of tuples. A tuple is an unordered set of attribute values.
A table is a two-dimensional representation of a relation in the form of rows (tuples) and columns (attributes). Each row in a table has the same set of columns. A relational database is a database that stores data in relations (tables). For example, a relational database could store information about company employees in an employee table, a department table, and a salary table.
See Also:
http://portal.acm.org/citation.cfm?id=362685 for an abstract and link to Codd's paperThe relational model is the basis for a relational database management system (RDBMS). Essentially, an RDBMS moves data into a database, stores the data, and retrieves it so that it can be manipulated by applications. An RDBMS makes a division between the following types of operations:
Logical operations
In this case, a user specifies what content is required. For example, a user requests the names of all employees or adds an employee record to a table.
Physical operations
In this case, the RDBMS determines how things should be done and carries out the operation. For example, after a user queries a table, the database may use an index to find the requested rows, read the data blocks into memory, and perform many other steps before returning a result to the user. The RDBMS stores and retrieves data so that physical operations are transparent to database applications.
Oracle Database is an RDBMS. An RDBMS that implements object-oriented features such as user-defined types, inheritance, and polymorphism is called an object-relational database management system (ORDBMS). Oracle Database has extended the relational model to an object-relational model, making it possible to store complex business models in a relational database.
The current version of Oracle Database is the result of over 30 years of innovative development. Highlights in the evolution of Oracle Database include the following:
Founding of Oracle
In 1977 Larry Ellison, Bob Miner, and Ed Oates started the consultancy Software Development Laboratories, which became Relational Software, Inc. (RSI). In 1983, RSI became Oracle Systems Corporation and then later Oracle Corporation.
First commercially available RDBMS
In 1979, RSI introduced Oracle V2 (Version 2) as the first commercially available SQL-based RDBMS, a landmark event in the history of relational databases.
Portable version of Oracle Database
Oracle Version 3, released in 1983, was the first relational database to run on mainframes, minicomputers, and PCs. The database was written in C, enabling the database to be ported to multiple platforms.
Enhancements to concurrency control, data distribution, and scalability
Version 4 introduced multiversion read consistency. Version 5, released in 1985, supported client/server computing and distributed database systems. Version 6 brought enhancements to disk I/O, row locking, scalability, and backup and recovery. Also, Version 6 introduced the first version of the PL/SQL language, a proprietary procedural extension to SQL.
PL/SQL stored program units
Oracle7, released in 1992, introduced PL/SQL stored procedures and triggers.
Objects and partitioning
Oracle8 was released in 1997 as the object-relational database, supporting many new data types. Additionally, Oracle8 supported partitioning of large tables.
Internet computing
Oracle8i Database, released in 1999, provided native support for internet protocols and server-side support for Java. Oracle8i was designed for internet computing, enabling the database to be deployed in a multitier environment.
Oracle Real Application Clusters (Oracle RAC)
Oracle9i Database introduced Oracle RAC in 2001, enabling multiple instances to access a single database simultaneously. Additionally, Oracle XML Database (Oracle XML DB) introduced the ability to store and query XML.
Grid computing
Oracle Database 10g introduced grid computing in 2003. This release enabled organizations to virtualize computing resources by building a grid infrastructure based on low-cost commodity servers. A key goal was to make the database self-managing and self-tuning. Oracle Automatic Storage Management (ASM) helped achieve this goal by virtualizing and simplifying database storage management.
Manageability, diagnosability, and availability
Oracle Database 11g, released in 2007, introduced a host of new features that enable administrators and developers to adapt quickly to changing business requirements. The key to adaptability is simplifying the information infrastructure by consolidating information and using automation wherever possible.
See Also:
http://www.oracle.com/technology/oramag/oracle/07-jul/o4730.html for a more detailed description of the evolution of Oracle DatabaseOne characteristic of an RDBMS is the independence of physical data storage from logical data structures. In Oracle Database, a database schema is a collection of logical data structures, or schema objects. A database schema is owned by a database user and has the same name as the user name.
Schema objects are user-created structures that directly refer to the data in the database. The database supports many types of schema objects, the most important of which are tables and indexes.
See Also:
Chapter 2, "Tables and Table Clusters" and Chapter 4, "Partitions, Views, and Other Schema Objects"A table describes an entity such as employees. You define a table with a table name, such as employees, and set of columns. In general, you give each column a name, a data type, and a width when you create the table.
A table is a set of rows. A column identifies an attribute of the entity described by the table, whereas a row identifies an instance of the entity. For example, attributes of the employees entity correspond to columns for employee ID and last name. A row identifies a specific employee.
You can optionally specify rules for each column of a table. These rules are called integrity constraints. One example is a NOT NULL integrity constraint. This constraint forces the column to contain a value in every row.
An index is an optional data structure created on one or more columns of a table. Indexes can increase the performance of data retrieval. When processing a request, the database can use available indexes to locate the requested rows efficiently. Indexes are useful when applications often query a specific row or range of rows.
Indexes are logically and physically independent of the data. Thus, indexes can be dropped and created with no effect on the tables or other indexes. All applications continue to function when an index is dropped.
See Also:
"Overview of Indexes"A general requirement for a DBMS is to adhere to industry accepted standards for a data access language.
SQL is a set-based declarative language that provides an interface to an RDBMS such as Oracle Database. In contrast to procedural languages such as C, which describe how things should be done, SQL is nonprocedural and describes what should be done. Users specify the result that they want (for example, the names of current employees), not how to derive it. SQL is the ANSI standard language for relational databases.
All operations on the data in an Oracle database are performed using SQL statements. For example, you use SQL to create tables and query and modify data in tables. A SQL statement can be thought of as a very simple, but powerful, computer program or instruction. A SQL statement is a string of SQL text such as the following:
SELECT first_name, last_name FROM employees;
SQL statements enable you to perform the following tasks:
Query data
Insert, update, and delete rows in a table
Create, replace, alter, and drop objects
Control access to the database and its objects
Guarantee database consistency and integrity
SQL unifies the preceding tasks in one consistent language. Oracle SQL is an implementation of the ANSI standard. Oracle SQL supports numerous features that extend beyond standard SQL.
See Also:
Chapter 7, "SQL"Oracle Database has a procedural extension to SQL called PL/SQL. The PL/SQL language is integrated with Oracle Database, enabling you to use all of the Oracle Database SQL statements, functions, and data types. You can use PL/SQL to control the flow of a SQL program, use variables, and write error-handling procedures.
A primary benefit of PL/SQL is the ability to store application logic in the database itself. A procedure or function is a schema object that consists of a set of SQL statements and other PL/SQL constructs, grouped together, stored in the database, and run as a unit to solve a specific problem or perform a set of related tasks. The principal benefit of server-side programming is that built-in functionality can be deployed anywhere.
Oracle Database can also store program units written in Java. A Java stored procedure is a Java method published to SQL and stored in the database for general use. You can call existing PL/SQL programs from Java and Java programs from PL/SQL.
Oracle Database is designed as a multiuser database. The database must ensure that multiple users can work concurrently without corrupting one another's data.
An RDBMS must be able to group SQL statements so that they are either all committed, which means they are applied to the database, or all rolled back, which means they are undone. A transaction is a logical, atomic unit of work that contains one or more SQL statements.
An illustration of the need for transactions is a funds transfer from a savings account to a checking account. The transfer consists of the following separate operations:
Decrease the savings account.
Increase the checking account.
Record the transaction in the transaction journal.
Oracle Database guarantees that all three operations succeed or fail as a unit. For example, if a hardware failure prevents a statement in the transaction from executing, then the other statements must be rolled back.
Transactions are one of the features that sets Oracle Database apart from a file system. If you perform an atomic operation that updates several files, and if the system fails halfway through, then the files will not be consistent. In contrast, a transaction moves an Oracle database from one consistent state to another. The basic principle of a transaction is "all or nothing": an atomic operation succeeds or fails as a whole.
See Also:
Chapter 9, "Transactions"A requirement of a multiuser RDBMS is the control of concurrency, which is the simultaneous access of the same data by multiple users. Without concurrency controls, users could change data improperly, compromising data integrity. For example, one user could update a row while a different user simultaneously updates it.
If multiple users access the same data, then one way of managing concurrency is to make users wait. However, the goal of a DBMS is to reduce wait time so it is either nonexistent or negligible. All SQL statements that modify data must proceed with as little interference as possible. Destructive interactions, which are interactions that incorrectly update data or alter underlying data structures, must be avoided.
Oracle Database uses locks to control concurrent access to data. A lock is a mechanism that prevents destructive interaction between transactions accessing a shared resource. Locks help ensure data integrity while allowing maximum concurrent access to data.
In Oracle Database, each user must see a consistent view of the data, including visible changes made by a user's own transactions and committed transactions of other users. For example, the database must prevent dirty reads, which occur when one transaction sees uncommitted changes made by another concurrent transaction.
Oracle Database always enforces statement-level read consistency, which guarantees that the data returned by a single query is committed and consistent with respect to a single point in time. Depending on the transaction isolation level, this point is the time at which the statement was opened or the time the transaction began. The Flashback Query feature enables you to specify this point in time explicitly.
The database can also provide read consistency to all queries in a transaction, known as transaction-level read consistency. In this case, each statement in a transaction sees data from the same point in time, which is the time at which the transaction began.
See Also:
Oracle Database Advanced Application Developer's Guide to learn about Flashback Query
A database server is the key to information management. In general, a server reliably manages a large amount of data in a multiuser environment so that users can concurrently access the same data. A database server also prevents unauthorized access and provides efficient solutions for failure recovery.
An Oracle database server consists of a database and at least one database instance (commonly referred to as simply an instance). Because an instance and a database are so closely connected, the term Oracle database is sometimes used to refer to both instance and database. In the strictest sense the terms have the following meanings:
Database
A database is a set of files, located on disk, that store data. These files can exist independently of a database instance.
Database instance
An instance is a set of memory structures that manage database files. The instance consists of a shared memory area, called the system global area (SGA), and a set of background processes. An instance can exist independently of database files.
Figure 1-1 shows a database and its instance. For each user connection to the instance, the application is run by a client process. Each client process is associated with its own server process. The server process has its own private session memory, known as the program global area (PGA).
A database can be considered from both a physical and logical perspective. Physical data is data viewable at the operating system level. For example, operating system utilities such as the Linux ls and ps can list database files and processes. Logical data such as a table is meaningful only for the database. A SQL statement can list the tables in an Oracle database, but an operating system utility cannot.
The database has physical structures and logical structures. Because the physical and logical structures are separate, the physical storage of data can be managed without affecting access to logical storage structures. For example, renaming a physical database file does not rename the tables whose data is stored in this file.
An essential task of a relational database is data storage. This section briefly describes the physical and logical storage structures used by Oracle Database.
The physical database structures are the files that store the data. When you execute the SQL command CREATE DATABASE, the following files are created:
Data files
Every Oracle database has one or more physical data files, which contain all the database data. The data of logical database structures, such as tables and indexes, is physically stored in the data files.
Control files
Every Oracle database has a control file. A control file contains metadata specifying the physical structure of the database, including the database name and the names and locations of the database files.
Online redo log files
Every Oracle Database has an online redo log, which is a set of two or more online redo log files. An online redo log is made up of redo entries (also called redo records), which record all changes made to data.
Many other files are important for the functioning of an Oracle database server. These files include parameter files and diagnostic files. Backup files and archived redo log files are offline files important for backup and recovery.
This section discusses logical storage structures. The following logical storage structures enable Oracle Database to have fine-grained control of disk space use:
Data blocks
At the finest level of granularity, Oracle Database data is stored in data blocks. One data block corresponds to a specific number of bytes on disk.
Extents
An extent is a specific number of logically contiguous data blocks, obtained in a single allocation, used to store a specific type of information.
Segments
A segment is a set of extents allocated for a user object (for example, a table or index), undo data, or temporary data.
Tablespaces
A database is divided into logical storage units called tablespaces. A tablespace is the logical container for a segment. Each tablespace contains at least one data file.
See Also:
Chapter 12, "Logical Storage Structures"An Oracle database uses memory structures and processes to manage and access the database. All memory structures exist in the main memory of the computers that constitute the RDBMS.
When applications connect to an Oracle Database, they are connected to a database instance. The instance services applications by allocating other memory areas in addition to the SGA, and starting other processes in addition to background processes.
A process is a mechanism in an operating system that can run a series of steps. Some operating systems use the terms job, task, or thread. For the purpose of this discussion, a thread is equivalent to a process. An Oracle database instance has the following types of processes:
Client processes
These processes are created and maintained to run the software code of an application program or an Oracle tool. Most environments have separate computers for client processes.
Background processes
These processes consolidate functions that would otherwise be handled by multiple Oracle Database programs running for each client process. Background processes asynchronously perform I/O and monitor other Oracle Database processes to provide increased parallelism for better performance and reliability.
Server processes
These processes communicate with client processes and interact with Oracle Database to fulfill requests.
Oracle processes include server processes and background processes. In most environments, Oracle processes and client processes run on separate computers.
See Also:
Chapter 15, "Process Architecture"Oracle Database creates and uses memory structures for purposes such as memory for program code, data shared among users, and private data areas for each connected user. The following memory structures are associated with an instance:
System Global Area (SGA)
The SGA is a group of shared memory structures that contain data and control information for one database instance. Examples of SGA components include cached data blocks and shared SQL areas.
Program Global Areas (PGA)
A PGA is a memory region that contain data and control information for a server or background process. Access to the PGA is exclusive to the process. Each server process and background process has its own PGA.
See Also:
Chapter 14, "Memory Architecture"To take full advantage of a given computer system or network, Oracle Database enables processing to be split between the database server and the client programs. The computer running the RDBMS handles the database server responsibilities while the computers running the applications handle the interpretation and display of data.
The application architecture refers to the computing environment in which a database application connects to an Oracle database. The two most common database architectures are client/server and multitier.
In a client/server architecture, the client application initiates a request for an operation to be performed on the database server. The server runs Oracle Database software and handles the functions required for concurrent, shared data access. The server receives and processes requests that originate from clients.
In a traditional multitier architecture, one or more application servers perform parts of the operation. An application server contains a large part of the application logic, provides access to the data for the client, and performs some query processing, thus lessening the load on the database. The application server can serve as an interface between clients and multiple databases and provide an additional level of security.
Service-oriented architecture (SOA) is a multitier architecture in which application functionality is encapsulated in services. SOA services are usually implemented as Web services. Web services can be accessed with the HTTP protocol and are based on a set of XML-based open standards, such as Web Services Description Language (WSDL) and SOAP. Oracle Database can act as a Web service provider in a traditional multitier or SOA environment.
See Also:
Oracle XML DB Developer's Guide for more information about using Web services with the database
Oracle Net Services is the interface between the database and the network communication protocols that facilitate distributed processing and distributed databases. Communication protocols define the way that data is transmitted and received on a network. Oracle Net Services supports communications on all major network protocols, including TCP/IP, HTTP, FTP, and WebDAV.
Oracle Net, a component of Oracle Net Services, establishes and maintains a network session from a client application to a database server. After a network session is established, Oracle Net acts as the data courier for both the client application and the database server, exchanging messages between them. Oracle Net can perform these jobs because it is located on each computer in the network.
An important component of Net Services is the Oracle Net Listener (called the listener), which is a separate process that runs on the database server or elsewhere in the network. Client applications can send connection requests to the listener, which manages the traffic of these requests to the database server. When a connection is established, the client and database communicate directly.
The most common ways to configure an Oracle database to service client requests are:
Dedicated server architecture
Each client process connects to a dedicated server process. The server process is not shared by any other client for the duration of the client's session. Each new session is assigned a dedicated server process.
Shared server architecture
The database uses a pool of shared processes for multiple sessions. A client process communicates with a dispatcher, which is a process that enables many clients to connect to the same database instance without the need for a dedicated server process for each client.
See Also:
Oracle Database Net Services Administrator's Guide for more information about network connections
Oracle XML DB Developer's Guide for information about using WebDAV with the database
This section explains how this manual should be read and where it fits into the Oracle Database documentation set as a whole.
To a new user, the Oracle Database documentation library can seem daunting. Not only are there are over 175 books, but many of these books are several hundred pages long. However, the documentation is designed with specific access paths to ensure that users are able to find the information they need as efficiently as possible.
The documentation set is divided into three layers or groups: basic, intermediate, and advanced. Users begin with the books in the basic group (Oracle Database 2 Day DBA, Oracle Database 2 Day Developer's Guide, or this manual), proceed to the books in the intermediate group (the 2 Day + series), and finally to the advanced books, which include the remainder of the documentation set.
Technical users who are new to Oracle Database begin by reading one or more books in the basic group from cover to cover. Each book in this group is designed to be read in two days. In addition to this manual, the basic group includes:
This book is a task-based DBA quick start guide that teaches you how to perform day-to-day database administrative tasks. It teaches you how to perform all common administrative tasks needed to keep the database operational, including how to perform basic troubleshooting and performance monitoring activities.
Oracle Database 2 Day Developer's Guide
This book is a task-based database developer quick start guide that explains how to use the basic features of Oracle Database through SQL and PL/SQL.
The books in the basic group are closely related, which is reflected in the number of cross-references. For example, Oracle Database Concepts frequently sends users to a 2 Day book to learn how to perform a task based on a concept. The 2 Day books frequently references Oracle Database Concepts for conceptual background about a task.
The next step up from the basic group is the intermediate group. The books in this group are prefixed with the word 2 Day + because they expand on and assume information contained in the 2 Day books. These books cover topics in more depth than was possible in the basic books, or cover topics of special interest. As shown in Table 1-1, the 2 Day + books are divided into books for DBAs and developers.
Table 1-1 Intermediate Group: 2 Day + Guides
| Database Administrators | Database Developers |
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Oracle Database 2 Day + Application Express Developer's Guide |
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Oracle Database 2 Day + Data Replication and Integration Guide |
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The next step up from the intermediate group is the advanced group. These books are intended for expert users who require more detailed information about a particular topic than can be provided by the 2 Day + books. Essential reference books in the advanced group include:
Oracle Database SQL Language Reference
This book is the definitive source of information about Oracle SQL.
The book is the definitive source of information about initialization parameters, data dictionary views, and dynamic performance views.
The advanced guides are too numerous to list in this section. Table 1-2 lists guides that are used by the majority of expert DBAs and developers at one time or another.
Table 1-2 Advanced Group
| Database Administrators | Database Developers |
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Oracle Real Application Clusters Administration and Deployment Guide |
Other advanced guides required by a particular user depend on the area of responsibility of this user. For example, a security officer will naturally refer to the Oracle Database Security Guide.
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