| Oracle® Database Concepts 10g Release 1 (10.1) Part Number B10743-01 |
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| Oracle® Database Concepts 10g Release 1 (10.1) Part Number B10743-01 |
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View PDF |
This chapter provides an overview of the Oracle database server. The topics include:
Oracle Database Application Development
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Note: This book contains information relating to both Oracle Database Standard Edition and Oracle Database Enterprise Edition. Some of the features and options documented in this chapter are available only if you have purchased the Oracle Database Enterprise Edition. See Oracle Database New Features for information about the differences between Oracle Database Standard Edition and Oracle Database Enterprise Edition. |
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 database has logical structures and physical structures. Because the physical and logical structures are separate, the physical storage of data can be managed without affecting the access to logical storage structures.
The section contains the following topics:
The Oracle grid architecture pools large numbers of servers, storage, and networks into a flexible, on-demand computing resource for enterprise computing needs. The grid computing infrastructure continually analyzes demand for resources and adjusts supply accordingly.
For example, you could run different applications on a grid of several linked database servers. When reports are due at the end of the month, the database administrator could automatically provision more servers to that application to handle the increased demand.
Grid computing uses sophisticated workload management that makes it possible for applications to share resources across many servers. Data processing capacity can be added or removed on demand, and resources within a location can be dynamically provisioned. Web services can quickly integrate applications to create new business processes.
Grid computing offers high performance and scalability, because all computing resources can be flexibly allocated to applications as needed.
Oracle Database enables enterprise grid computing in the following ways:
Performance and scalability with low cost hardware clusters, like Itanium and Linux.
Reliability: Continuous availability of data and applications
Security and privacy: security features that lets you share enterprise grid resources with confidence that privacy is maintained
Self-management: Oracle infrastructure automates many functions so that a single administrator can manage hundreds of servers.
Distributed computing: Oracle has advanced integration features that allow applications and data to run anywhere in the network.
Difference between a cluster and a grid: Clustering is one technology used to create a grid infrastructure. Simple clusters have static resources for specific applications by specific owners. Grids, which can consist of multiple clusters, are dynamic resource pools shareable among many different applications and users. A grid does not assume that all servers in the grid are running the same set of applications. Applications can be scheduled and migrated across servers in the grid. Grids share resources from and among independent system owners.
At the highest level, the idea of grid computing is computing as a utility. In other words, you should not care where your data resides, or what computer processes your request. You should be able to request information or computation and have it delivered - as much as you want, and whenever you want. This is analogous to the way electric utilities work, in that you don't know where the generator is, or how the electric grid is wired, you just ask for electricity, and you get it. The goal is to make computing a utility, a commodity, and ubiquitous. Hence the name, The Grid. This view of utility computing is, of course, a "client side" view.
From the "server side", or behind the scenes, the grid is about resource allocation, information sharing, and high availability. Resource allocation ensures that all those that need or request resources are getting what they need, that resources are not standing idle while requests are going unserviced. Information sharing makes sure that the information users and applications need is available where and when it is needed. High availability features guarantee all the data and computation is always there, just like a utility company always provides electric power.
Oracle provides substantial grid computing technology, available today, that can help you capitalize on the grid. Oracle is the best on commodity clusters, a hardware platform that many believe would come to dominate the grid. Oracle possesses the key technology differentiators -- Oracle Real Application Clusters, Oracle Streams, Oracle Transportable Tablespaces -- for building the grid. Only Oracle delivers the operational characteristics -- portability, RAS, security, and scalability -- necessary for the grid.
Recently, ideas behind grid computing have evolved, due in large part to advances in hardware and networking technologies and the drop in costs associated with these technologies. New high-volume processors and inexpensive blade servers, for example, are extremely affordable compared to their high-end SMP predecessors. Instead of scavenging resources as in the older model of grid computing, a business could relatively cheaply build a blade server farm whose resources could be dynamically and automatically allocated to the areas of the enterprise that required the computing power.
In addition to operating on low-cost, off-the-shelf hardware that can be quickly assembled to create a relatively large-scale operation, this vision of grid computing allows for a high level of flexibility in meeting existing and future computing needs.
Oracle has been working for years on technologies that support and enable grid computing.
Computing resource provisioning is one of the most important capabilities of a grid. This enables computing resources to be dynamically provisioned to applications as required. Resources must be appropriately allocated based on business priorities and demand. Oracle provides a number features for computing resource provisioning, including:
Real Application Clusters. RAC is a cluster database with a shared cache architecture that runs on multiple machines, attached through a cluster interconnect and a shared storage subsystem. An Oracle RAC database not only appears like a single standard Oracle Database to users, but the same maintenance tools and practices used for a single Oracle Database can be used on the entire cluster. All standard backup and recovery operations, including the use of Recovery Manager, work transparently with RAC. All SQL operations, including data definition language and integrity constraints, are also identical for both configurations. The most important part of RAC, however, is the ability to manage your workload—to add nodes or relinquish nodes on demand—based on your business processing needs.
Automatic Storage Management. Oracle recommends using Automatic Storage Management (ASM) for your database files and a cluster file system for the Oracle home. ASM simplifies the administration of Oracle database files. Instead of managing many database files, ASM requires you to manage only a small number of disk groups. You can define a particular disk group as the default disk group for a database.
Oracle Resource Manager. Though Oracle Database is largely a self-managing database, Database Resource Manager allows resource administrators to influence how the Oracle database resources are allocated to users.
Oracle Scheduler. Oracle Scheduler provides many capabilities to schedule and perform business and IT tasks, called jobs, in a grid.
Information provisioning means delivering information to users whenever they need it, regardless of where it resides on the grid. To process information on any available resource, the grid must efficiently share information across distributed systems. The grid must also provide access to data residing on heterogeneous systems—database systems from multiple vendors and file systems. Oracle provides a broad set of features and tools for information provisioning on a grid, including the following:
Oracle Transportable Tablespaces. Transportable Tablespaces allows Oracle datafiles to be unplugged from a database, moved, or copied to another location, and then plugged into another database. Unplugging or plugging a datafile involves reading or loading only a small amount of metadata. Transportable Tablespaces also supports simultaneous mounting of read-only tablespaces by two or more databases.
Oracle Streams. Some data needs to be shared as it is created or changed, rather than occasionally shared in bulk. Oracle Streams can stream data between databases, nodes, or blade farms in a grid and can keep two or more copies in sync as updates are applied. It also provides a unified framework for information sharing, combining message queuing, replication, events, data warehouse loading, notifications, and publish/subscribe into a single technology.
A combination Streams/Transportable Tablespace feature enables a self-propelled database. With a single command, you can take a tablespace from one database, ship the tablespace to another database, reformat the tablespace if the second database is on a different operating system, plug this tablespace into the second database, and start syncing the tablespace with the changes happening in the first database. If the second database is on a grid, then you have just migrated your application to a grid with a single command.
Easy OCI/JDBC Install. With Oracle Database, any application using Oracle Call Interface (OCI) can easily install a small footprint version of the Oracle client files without installing and configuring the entire client. These applications include Java database connectivity (JDBC) type-2 driver applications. OCI and the JDBC type-2 driver (which requires OCI) can easily be installed by downloading a small subset of the Oracle client files and updating a few environment variables to point to the location of the downloaded libraries. This means your grid client doesn't need to install any Oracle software and yet you get easy, secure access to data from an Oracle database running on a grid.
Distributed SQL and Distributed Transactions. Oracle Distributed SQL allows grid users to efficiently access and integrate data stored in multiple Oracle and non-Oracle databases. Transparent remote data access with Distributed SQL allows grid users to run their applications against any other database without making any code change to the applications. While integrating data and managing transactions across multiple data stores, the Oracle database intelligently optimizes the execution plans to access data in the most efficient manner.
Ultra Large Database Support. For enabling Ultra Large Databases (ULDB), the Bigfile Tablespace feature allows Oracle Database to contain tablespaces made up of single large files rather than numerous smaller ones. This allows Oracle Database to utilize the ability of 64-bit systems to create and manage ultralarge files. The consequence of this is that Oracle Database can now scale up to 8 exabytes in size.
Because some of the key goals of a grid are to provide high availability, scalability, and service performance optimization with minimal costs and complexity, an integrated approach to grid management is necessary. These tools and features include:
Grid Management with Oracle Enterprise Manager. Enterprise Manager provides a simplified, centralized management framework for managing enterprise resources and analyzing a grid's performance. With Enterprise Manager, administrators can manage the grid environment through a Web browser throughout the system lifecycle, front to back, from any location on the network. With Oracle Database, this includes integrated management and monitoring of RAC databases as well as standby Data Guard systems, for failover or switchover scenarios.
The dynamic environment in a grid makes security extremely important. Oracle makes managing security easy for you by centralizing security management for a distributed enterprise using the Lightweight Directory Access Protocol (LDAP)-compliant Oracle Internet Directory (OID). In this regard, Enterprise User Security provides the ability to create and manage privileges of a user globally—across all enterprise databases. The enterprise user privilege administration is done in the OID, thus avoiding the need to create the same user in multiple databases across a grid. Additionally, you now can store a Secure Sockets Layer (SSL) Certificate in a smart card, for roaming access to the grid.
Virtual Private Database. VPD provides server-enforced, fine-grained access control and a secure application context that can be used in a grid setting to enable multiple customers, partners, or departments utilizing the same database to have secure access to mission-critical data. VPD enables per-user and per-customer data access within a single database, with the assurance of physical data separation. VPD is enabled by associating one or more security policies with tables or views.
Oracle Label Security. Oracle Label Security gives administrators an out-of-the-box row-level and column-level security solution for controlling access to data based on its sensitivity, eliminating the need to manually write such policies. Using the GUI tool Oracle Policy Manager, administrators can quickly create and assign Oracle Label Security policies to rows and columns within application tables. Moreover, Oracle Database adds integration of Oracle Label Security with OID, allowing policies to be managed centrally within a dynamically changing grid setting.
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.
Multiprocessing uses more 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.
An Oracle database system can easily take advantage of distributed processing by using its client/server architecture. In this architecture, the database system is divided into two parts: a front-end or a client, and a back-end or a server.
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 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.
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.
The following sections explain the physical database structures of an Oracle database, including datafiles, redo log files, and control files.
Every Oracle database has one or more physical datafiles. The datafiles contain all the database data. The data of logical database structures, such as tables and indexes, is physically stored in the datafiles allocated for a database.
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 (DBWn) background process.
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See Also: "Overview of the Oracle Instance" for more information about Oracle's memory and process structures |
Every Oracle database has a control file. A control file contains entries that specify the physical structure of the database. For example, it contains the following information:
Database name
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.
Every Oracle database has a set of two or more redo log files. The set of redo log files is collectively known as the redo log for the database. A redo log is made up of redo entries (also called redo records).
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.
The process of applying the redo log during a recovery operation is called rolling forward.
You can enable automatic archiving of the redo log. Oracle automatically archives log files when the database is in ARCHIVELOG mode.
Parameter 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.
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See Also:
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Each server and background process can write to an associated trace 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 file, or alert log, is a special trace file. The alert file of a database is a chronological log of messages and errors.
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.
The logical storage structures, including data blocks, extents, and segments, enable Oracle to have fine-grained control of disk space use.
A database is divided into logical storage units called tablespaces, which group related logical structures together. For example, tablespaces commonly group together all application objects to simplify some administrative operations.
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.
Every Oracle database contains a SYSTEM tablespace and a SYSAUX tablespace. Oracle creates them automatically when the database is created. The system default is to create a smallfile tablespace, which is the traditional type of Oracle tablespace. The SYSTEM and SYSAUX tablespaces are created as smallfile tablespaces.
Oracle also lets you create bigfile tablespaces up to 8 exabytes (8 million terabytes) in size. With Oracle-managed files, bigfile tablespaces make datafiles completely transparent for users. In other words, you can perform operations on tablespaces, rather than the underlying datafiles.
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.
At the finest level of granularity, Oracle database data is stored in data blocks. One data block corresponds to a specific number of bytes of physical database space on disk. The standard block size is specified by the DB_BLOCK_SIZE initialization parameter. In addition, you can specify up to five other block sizes. A database uses and allocates free database space in Oracle data blocks.
The next level of logical database space is an extent. An extent is a specific number of contiguous data blocks, obtained in a single allocation, used to store a specific type of information.
Above extents, the level of logical database storage is a segment. A segment is a set of extents allocated for a certain logical structure. The following table describes the different types of segments.
| Segment | Description |
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| Data 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. |
| Index segment | Each index has an index segment that stores all of its data.
For a partitioned index, each partition has an index segment. |
| Temporary segment | Temporary segments are created by Oracle when a SQL statement needs a temporary database area to complete execution. When the statement finishes execution, the extents in the temporary segment are returned to the system for future use. |
| Rollback segment | If you are operating in automatic undo management mode, then the database server manages undo space using tablespaces. Oracle recommends that you use automatic undo management.
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 transactions 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 |
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.
A schema is a collection of database objects. A schema is owned by a database user and has the same name as that user. Schema objects are the logical structures that directly refer to the database's data. Schema objects include structures like tables, views, and indexes. (There is no relationship between a tablespace and a schema. Objects in the same schema can be in different tablespaces, and a tablespace can hold objects from different schemas.)
Some of the most common schema objects are defined in the following section.
Tables are the basic unit of data storage in an Oracle database. Database tables hold all user-accessible data. Each table has columns and rows. A table that has an employee database, for example, can have a column called employee number, and each row in that column is an employee's number.
Indexes are optional structures associated with tables. Indexes can be created to increase the performance of data retrieval. Just as the index in this manual helps you quickly locate specific information, an Oracle index provides an access path to table data.
When processing a request, Oracle can use some or all of the available indexes to locate the requested rows efficiently. Indexes are useful when applications frequently query a table for a range of rows (for example, all employees with a salary greater than 1000 dollars) or a specific row.
Indexes are created on one or more columns of a table. After it is created, an index is automatically maintained and used by Oracle. Changes to table data (such as adding new rows, updating rows, or deleting rows) are automatically incorporated into all relevant indexes with complete transparency to the users.
Views are customized presentations of data in one or more tables or other views. A view can also be considered a stored query. Views do not actually contain data. Rather, they derive their data from the tables on which they are based, referred to as the base tables of the views.
Like tables, views can be queried, updated, inserted into, and deleted from, with some restrictions. All operations performed on a view actually affect the base tables of the view.
Views provide an additional level of table security by restricting access to a predetermined set of rows and columns of a table. They also hide data complexity and store complex queries.
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.
Like indexes, clusters do not affect application design. Whether a table is part of a cluster is transparent to users and to applications. Data stored in a clustered table is accessed by SQL in the same way as data stored in a nonclustered table.
Each Oracle database has a data dictionary. An Oracle data dictionary is a set of tables and views that are used as a read-only reference about the database. For example, a data dictionary stores information about both the logical and physical structure of the database. A data dictionary also stores the following information:
The valid users of an Oracle database
Information about integrity constraints defined for tables in the database
The amount of space allocated for a schema object and how much of it is in use
A data dictionary is created when a database is created. To accurately reflect the status of the database at all times, the data dictionary is automatically updated by Oracle in response to specific actions, such as when the structure of the database is altered. The database relies on the data dictionary to record, verify, and conduct ongoing work. For example, during database operation, Oracle reads the data dictionary to verify that schema objects exist and that users have proper access to them.
An Oracle database server consists of an Oracle database and an Oracle instance. Every time a database is started, a system global area (SGA) is allocated and Oracle background processes are started. The combination of the background processes and memory buffers is called an Oracle instance.
Some hardware architectures (for example, shared disk systems) enable multiple computers to share access to data, software, or peripheral devices. Real Application Clusters (RAC) takes advantage of such architecture by running multiple instances that share a single physical database. In most applications, RAC enables access to a single database by users on multiple machines with increased performance.
An Oracle database server 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 database system. Processes are jobs that work in the memory of these computers.
Oracle creates and uses memory structures to complete several jobs. For example, memory stores program code being run and data shared among users. Two basic memory structures are associated with Oracle: the system global area and the program global area. The following subsections explain each in detail.
The System Global Area (SGA) is a shared memory region that contains data and control information for one Oracle instance. Oracle allocates the SGA when an instance starts and deallocates it when the instance shuts down. Each instance has its own SGA.
Users currently connected to an Oracle database share the data in the SGA. For optimal performance, the entire SGA should be as large as possible (while still fitting in real memory) to store as much data in memory as possible and to minimize disk I/O.
The information stored in the SGA is divided into several types of memory structures, including the database buffers, redo log buffer, and the shared pool.
Database buffers store the most recently used blocks of data. The set of database buffers in an instance is the database buffer cache. The buffer cache contains modified as well as unmodified blocks. Because the most recently (and often, the most frequently) used data is kept in memory, less disk I/O is necessary, and performance is improved.
The redo log buffer 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, which is used if database recovery is necessary. The size of the redo log is static.
The shared pool 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. A shared SQL area contains information such as the parse tree and execution plan for the corresponding statement. A single shared SQL area is used by multiple applications that issue the same statement, leaving more shared memory for other uses.
A cursor is a handle or name for a private SQL area in which a parsed statement and other information for processing the statement are kept. (Oracle Call Interface, OCI, refers to these as statement handles.) Although most Oracle users rely on automatic cursor handling of Oracle utilities, the programmatic interfaces offer application designers more control over cursors.
For example, in precompiler application development, a cursor is a named resource available to a program and can be used specifically to parse SQL statements embedded within the application. Application developers can code an application so it controls the phases of SQL statement execution and thus improves application performance.
The Program Global Area (PGA) is 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.
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 database system. Processes are jobs that work in the memory of these computers.
The architectural features discussed in this section enable the Oracle database to support:
Many users concurrently accessing a single database
The high performance required by concurrent multiuser, multiapplication database systems
Oracle creates a set of background processes for each instance. The background processes consolidate functions that would otherwise be handled by multiple Oracle programs running for each user process. They asynchronously perform I/O and monitor other Oracle process to provide increased parallelism for better performance and reliability.
There are numerous background processes, and each Oracle instance can use several background processes.
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See Also: "Background Processes" for more information on some of the most common background processes |
A process is a "thread of control" or a mechanism in an operating system that can run a series of steps. Some operating systems use the terms job or task. A process generally has its own private memory area in which it runs.
An Oracle database server has two general types of processes: user processes and Oracle processes.
User processes are created and maintained to run the software code of an application program (such as an OCI or OCCI program) or an Oracle tool (such as Enterprise Manager). User processes also manage communication with the server process through the program interface, which is described in a later section.
Oracle processes are invoked by other processes to perform functions on behalf of the invoking process.
Oracle creates server processes to handle requests from connected user processes. A server process communicates with the user process and interacts with Oracle to carry out requests from the associated user process. For example, if a user queries some data not already in the database buffers of the SGA, then the associated server process reads the proper data blocks from the datafiles into the SGA.
Oracle can be configured to vary the number of user processes for each server process. In a dedicated server configuration, a server process handles requests for a single user process. A shared server configuration lets many user processes share a small number of server processes, minimizing the number of server processes and maximizing the use of available system resources.
On some systems, the user and server processes are separate, while on others they are combined into a single process. If a system uses the shared server or if the user and server processes run on different machines, then the user and server processes must be separate. Client/server systems separate the user and server processes and run them on different machines.
This section describes Oracle Net Services, as well as how to start up the database.
Oracle Net Services is Oracle's mechanism for interfacing with the communication protocols used by the networks 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.
Using Oracle Net Services, application developers do not need to be concerned with supporting network communications in a database application. If a new protocol is used, then the database administrator makes some minor changes, while the application requires no modifications and continues to function.
Oracle Net, a component of Oracle Net Services, enables a network session from a client application to an Oracle database server. Once a network session is established, Oracle Net acts as the data courier for both the client application and the database server. It establishes and maintains the connection between the client application and database server, as well as exchanges messages between them. Oracle Net can perform these jobs because it is located on each computer in the network.
The three steps to starting an Oracle database and making it available for systemwide use are:
Start an instance.
Mount the database.
Open the database.
A database administrator can perform these steps using the SQL*Plus STARTUP statement or Enterprise Manager. When Oracle starts an instance, it reads the server parameter file (SPFILE) or initialization parameter file to determine the values of initialization parameters. Then, it allocates an SGA, and creates background processes.
The following example describes the most basic level of operations that Oracle performs. This illustrates an Oracle configuration where the user and associated server process are on separate machines (connected through a network).
An instance has started on the computer running Oracle (often called the host or database server).
A computer running an application (a local machine or client workstation) runs the application in a user process. The client application attempts to establish a connection to the server using the proper Oracle Net Services driver.
The server is running the proper Oracle Net Services driver. The server detects the connection request from the application and creates a dedicated server process on behalf of the user process.
The user runs a SQL statement and commits the transaction. For example, the user changes a name in a row of a table.
The server process receives the statement and checks the shared pool for any shared SQL area that contains a similar SQL statement. If a shared SQL area is found, then the server process checks the user's access privileges to the requested data, and the previously existing shared SQL area is used to process the statement. If not, then a new shared SQL area is allocated for the statement, so it can be parsed and processed.
The server process retrieves any necessary data values from the actual datafile (table) or those stored in the SGA.
The server process modifies data in the system global area. The DBWn process writes modified blocks permanently to disk when doing so is efficient. Because the transaction is committed, the LGWR process immediately records the transaction in the redo log file.
If the transaction is successful, then the server process sends a message across the network to the application. If it is not successful, then an error message is transmitted.
Throughout this entire procedure, the other background processes run, watching for conditions that require intervention. In addition, the database server manages other users' transactions and prevents contention between transactions that request the same data.
This section contains the following topics:
Oracle includes several software mechanisms to fulfill the following important requirements of an information management system:
Data concurrency of a multiuser system must be maximized.
Data must be read and modified in a consistent fashion. The data a user is viewing or changing is not changed (by other users) until the user is finished with the data.
High performance is required for maximum productivity from the many users of the database system.
This contains the following sections:
A primary concern of a multiuser database management system is how to control concurrency, which is the simultaneous access of the same data by many users. Without adequate concurrency controls, data could be updated or changed improperly, compromising data integrity.
One way to manage data concurrency is to make each user wait for a turn. The goal of a database management system is to reduce that wait so it is either nonexistent or negligible to each user. All data manipulation language statements should proceed with as little interference as possible, and destructive interactions between concurrent transactions must be prevented. Destructive interaction is any interaction that incorrectly updates data or incorrectly alters underlying data structures. Neither performance nor data integrity can be sacrificed.
Oracle resolves such issues by using various types of locks and a multiversion consistency model. These features are based on the concept of a transaction. It is the application designer's responsibility to ensure that transactions fully exploit these concurrency and consistency features.
Read consistency, as supported by Oracle, does the following:
Guarantees that the set of data seen by a statement is consistent with respect to a single point in time and does not change during statement execution (statement-level read consistency)
Ensures that readers of database data do not wait for writers or other readers of the same data
Ensures that writers of database data do not wait for readers of the same data
Ensures that writers only wait for other writers if they attempt to update identical rows in concurrent transactions
The simplest way to think of Oracle's implementation of read consistency is to imagine each user operating a private copy of the database, hence the multiversion consistency model.
To manage the multiversion consistency model, Oracle must create a read-consistent set of data when a table is queried (read) and simultaneously updated (written). When an update occurs, the original data values changed by the update are recorded in the database undo records. As long as this update remains part of an uncommitted transaction, any user that later queries the modified data views the original data values. Oracle uses current information in the system global area and information in the undo records to construct a read-consistent view of a table's data for a query.
Only when a transaction is committed are the changes of the transaction made permanent. Statements that start after the user's transaction is committed only see the changes made by the committed transaction.
The transaction is key to Oracle's strategy for providing read consistency. This unit of committed (or uncommitted) SQL statements:
Dictates the start point for read-consistent views generated on behalf of readers
Controls when modified data can be seen by other transactions of the database for reading or updating
By default, Oracle guarantees statement-level read consistency. The set of data returned by a single query is consistent with respect to a single point in time. However, in some situations, you might also require transaction-level read consistency. This is the ability to run multiple queries within a single transaction, all of which are read-consistent with respect to the same point in time, so that queries in this transaction do not see the effects of intervening committed transactions. If you want to run a number of queries against multiple tables and if you are not doing any updating, you prefer a read-only transaction.
Oracle also uses locks to control concurrent access to data. When updating information, the data server holds that information with a lock until the update is submitted or committed. Until that happens, no one else can make changes to the locked information. This ensures the data integrity of the system.
Oracle provides unique non-escalating row-level locking. Unlike other data servers that "escalate" locks to cover entire groups of rows or even the entire table, Oracle always locks only the row of information being updated. Because Oracle includes the locking information with the actual rows themselves, Oracle can lock an unlimited number of rows so users can work concurrently without unnecessary delays.
Oracle locking is performed automatically and requires no user action. Implicit locking occurs for SQL statements as necessary, depending on the action requested. Oracle's lock manager automatically locks table data at the row level. By locking table data at the row level, contention for the same data is minimized.
Oracle's lock manager maintains several different types of row locks, depending on what type of operation established the lock. The two general types of locks are exclusive locks and share locks. Only one exclusive lock can be placed on a resource (such as a row or a table); however, many share locks can be placed on a single resource. Both exclusive and share locks always allow queries on the locked resource but prohibit other activity on the resource (such as updates and deletes).
Database administrators occasionally need isolation from concurrent non-database administrator actions, that is, isolation from concurrent non-database administrator transactions, queries, or PL/SQL statements. One way to provide such isolation is to shut down the database and reopen it in restricted mode. You could also put the system into quiesced state without disrupting users. In quiesced state, the database administrator can safely perform certain actions whose executions require isolation from concurrent non-DBA users.
Real Application Clusters (RAC) comprises several Oracle instances running on multiple clustered machines, which communicate with each other by means of a so-called interconnect. RAC uses cluster software to access a shared database that resides on shared disk. RAC combines the processing power of these multiple interconnected computers to provide system redundancy, near linear scalability, and high availability. RAC also offers significant advantages for both OLTP and data warehouse systems and all systems and applications can efficiently exploit clustered environments.
You can scale applications in RAC environments to meet increasing data processing demands without changing the application code. As you add resources such as nodes or storage, RAC extends the processing powers of these resources beyond the limits of the individual components.
Oracle provides unique portability across all major platforms and ensures that your applications run without modification after changing platforms. This is because the Oracle code base is identical across platforms, so you have identical feature functionality across all platforms, for complete application transparency. Because of this portability, you can easily upgrade to a more powerful server as your requirements change.
People who administer the operation of an Oracle database system, known as database administrators (DBAs), are responsible for creating Oracle databases, ensuring their smooth operation, and monitoring their use. In addition to the many alerts and advisors Oracle provides, Oracle also offers the following features:
Oracle Database provides a high degree of self-management - automating routine DBA tasks and reducing complexity of space, memory, and resource administration. Oracle self-managing database features include the following: automatic undo management, dynamic memory management, Oracle-managed files, mean time to recover, free space management, multiple block sizes, and Recovery Manager (RMAN).
Enterprise Manager is a system management tool that provides an integrated solution for centrally managing your heterogeneous environment. Combining a graphical console, Oracle Management Servers, Oracle Intelligent Agents, common services, and administrative tools, Enterprise Manager provides a comprehensive systems management platform for managing Oracle products.
From the client interface, the Enterprise Manager Console, you can perform the following tasks:
Administer the complete Oracle environment, including databases, iAS servers, applications, and services
Diagnose, modify, and tune multiple databases
Schedule tasks on multiple systems at varying time intervals
Monitor database conditions throughout the network
Administer multiple network nodes and services from many locations
Share tasks with other administrators
Group related targets together to facilitate administration tasks
Launch integrated Oracle and third-party tools
Customize the display of an Enterprise Manager administrator
SQL*Plus is a tool for entering and running ad-hoc database statements. It lets you run SQL statements and PL/SQL blocks, and perform many additional tasks as well.
Automatic Storage Management automates and simplifies the layout of datafiles, control files, and log files. Database files are automatically distributed across all available disks, and database storage is rebalanced whenever the storage configuration changes. It provides redundancy through the mirroring of database files, and it improves performance by automatically distributing database files across all available disks. Rebalancing of the database's storage automatically occurs whenever the storage configuration changes.
To help simplify management tasks, as well as providing a rich set of functionality for complex scheduling needs, Oracle provides a collection of functions and procedures in the DBMS_SCHEDULER package. Collectively, these functions are called the Scheduler, and they are callable from any PL/SQL program.
The Scheduler lets database administrators and application developers control when and where various tasks take place in the database environment. For example, database administrators can schedule and monitor database maintenance jobs such as backups or nightly data warehousing loads and extracts.
Traditionally, the operating systems regulated resource management among the various applications running on a system, including Oracle databases. The Database Resource Manager controls the distribution of resources among various sessions by controlling the execution schedule inside the database. By controlling which sessions run and for how long, the Database Resource Manager can ensure that resource distribution matches the plan directive and hence, the business objectives.
In every database system, the possibility of a system or hardware failure always exists. If a failure occurs and affects the database, then the database must be recovered. The goals after a failure are to ensure that the effects of all committed transactions are reflected in the recovered database and to return to normal operation as quickly as possible while insulating users from problems caused by the failure.
Oracle provides various mechanisms for the following:
Database recovery required by different types of failures
Flexible recovery operations to suit any situation
Availability of data during backup and recovery operations so users of the system can continue to work
Several circumstances can halt the operation of an Oracle database. The most common types of failure are described in the following table.
Oracle provides for complete media recovery from all possible types of hardware failures, including disk failures. Options are provided so that a database can be completely recovered or partially recovered to a specific point in time.
If some datafiles are damaged in a disk failure but most of the database is intact and operational, the database can remain open while the required tablespaces are individually recovered. Therefore, undamaged portions of a database are available for normal use while damaged portions are being recovered.
Oracle uses several structures to provide complete recovery from an instance or disk failure: the redo log, undo records, a control file, and database backups.
The redo log is 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 the online redo log and the archived redo log.
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Note: Because the online redo log is always online, as opposed to an archived copy of a redo log, thus it is usually referred to as simply "the 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 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. LGWR writes redo entries continually, and it also writes a commit record every time a user process commits a transaction.
Optionally, filled online redo files can be manually or automatically archived before being reused, creating archived redo logs. To enable or disable archiving, set the database in one of the following modes:
ARCHIVELOG: The filled online redo log files are archived before they are reused in the cycle.
NOARCHIVELOG: The filled online redo log files are not archived.
In ARCHIVELOG mode, the database can be completely recovered from both instance and disk failure. The database can also be backed up while it is open and available for use. However, additional administrative operations are required to maintain the archived redo log.
If the database redo log operates in NOARCHIVELOG mode, then the database can be completely recovered from instance failure, but not from disk failure. Also, the database can be backed up only while it is completely closed. Because no archived redo log is created, no extra work is required by the database administrator.
Undo records are stored in undo tablespaces. Oracle uses the undo data for a variety of purposes, including accessing before-images of blocks changed in uncommitted transactions. During database recovery, Oracle applies all changes recorded in the redo log and then uses undo information to roll back any uncommitted transactions.
The control files include information about the file structure of the database and the current log sequence number being written by LGWR. During normal recovery procedures, the information in a control file guides the automatic progression of the recovery operation.
Because one or more files can be physically damaged as the result of a disk failure, media recovery requires the restoration of the damaged files from the most recent operating system backup of a database. You can either back up the database files with Recovery Manager (RMAN), or use operating system utilities. RMAN is 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.
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Computing environments configured to provide nearly full-time availability are known as high availability systems. Such systems typically have redundant hardware and software that makes the system available despite failures. Well-designed high availability systems avoid having single points-of-failure.
When failures occur, the fail over process moves processing performed by the failed component to the backup component. This process remasters systemwide resources, recovers partial or failed transactions, and restores the system to normal, preferably within a matter of microseconds. The more transparent that fail over is to users, the higher the availability of the system.
Oracle has a number of products and features that provide high availability in cases of unplanned downtime or planned downtime. These include Fast-Start Fault Recovery, Real Application Clusters, Recovery Manager (RMAN), backup and recovery solutions, Oracle Flashback, partitioning, Oracle Data Guard, LogMiner, multiplexed redo log files, online reorganization. These can be used in various combinations to meet specific high availability needs.
This section describes several business intelligence features.
A data warehouse is a relational database designed for query and analysis rather than for transaction processing. It usually contains historical data derived from transaction data, but it can include data from other sources. It separates analysis workload from transaction workload and enables an organization to consolidate data from several sources.
In addition to a relational database, a data warehouse environment includes an extraction, transportation, transformation, and loading (ETL) solution, an online analytical processing (OLAP) engine, client analysis tools, and other applications that manage the process of gathering data and delivering it to business users.
You must load your data warehouse regularly so that it can serve its purpose of facilitating business analysis. To do this, data from one or more operational systems must be extracted and copied into the warehouse. The process of extracting data from source systems and bringing it into the data warehouse is commonly called ETL, which stands for extraction, transformation, and loading.
A materialized view provides indirect access to table data by storing the results of a query in a separate schema object. Unlike an ordinary view, which does not take up any storage space or contain any data, a materialized view contains the rows resulting from a query against one or more base tables or views. A materialized view can be stored in the same database as its base tables or in a different database.
Materialized views stored in the same database as their base tables can improve query performance through query rewrites. Query rewrites are particularly useful in a data warehouse environment.
Data warehousing environments typically have large amounts of data and ad hoc queries, but a low level of concurrent database manipulation language (DML) transactions. For such applications, bitmap indexing provides:
Reduced response time for large classes of ad hoc queries
Reduced storage requirements compared to other indexing techniques
Dramatic performance gains even on hardware with a relatively small number of CPUs or a small amount of memory
Efficient maintenance during parallel DML and loads
Fully indexing a large table with a traditional B-tree index can be prohibitively expensive in terms of space because the indexes can be several times larger than the data in the table. Bitmap indexes are typically only a fraction of the size of the indexed data in the table.
To reduce disk use and memory use (specifically, the buffer cache), you can store tables and partitioned tables in a compressed format inside the database. This often leads to a better scaleup for read-only operations. Table compression can also speed up query execution. There is, however, a slight cost in CPU overhead.
When Oracle runs SQL statements in parallel, multiple processes work together simultaneously to run a single SQL statement. By dividing the work necessary to run a statement among multiple processes, Oracle can run the statement more quickly than if only a single process ran it. This is called parallel execution or parallel processing.
Parallel execution dramatically reduces response time for data-intensive operations on large databases, because statement processing can be split up among many CPUs on a single Oracle system.
Oracle has many SQL operations for performing analytic operations in the database. These include ranking, moving averages, cumulative sums, ratio-to-reports, and period-over-period comparisons.
Application developers can use SQL online analytical processing (OLAP) functions for standard and ad-hoc reporting. For additional analytic functionality, Oracle OLAP provides multidimensional calculations, forecasting, modeling, and what-if scenarios. This enables developers to build sophisticated analytic and planning applications such as sales and marketing analysis, enterprise budgeting and financial analysis, and demand planning systems. Data can be stored in either relational tables or multidimensional objects.
Oracle OLAP provides the query performance and calculation capability previously found only in multidimensional databases to Oracle's relational platform. In addition, it provides a Java OLAP API that is appropriate for the development of internet-ready analytical applications. Unlike other combinations of OLAP and RDBMS technology, Oracle OLAP is not a multidimensional database using bridges to move data from the relational data store to a multidimensional data store. Instead, it is truly an OLAP-enabled relational database. As a result, Oracle provides the benefits of a multidimensional database along with the scalability, accessibility, security, manageability, and high availability of the Oracle database. The Java OLAP API, which is specifically designed for internet-based analytical applications, offers productive data access.
With Oracle Data Mining, data never leaves the database — the data, data preparation, model building, and model scoring results all remain in the database. This enables Oracle to provide an infrastructure for application developers to integrate data mining seamlessly with database applications. Some typical examples of the applications that data mining are used in are call centers, ATMs, ERM, and business planning applications. Data mining functions such as model building, testing, and scoring are provided through a Java API.
Partitioning addresses key issues in supporting very large tables and indexes by letting you decompose them into smaller and more manageable pieces called partitions. SQL queries and DML statements do not need to be modified in order to access partitioned tables. However, after partitions are defined, DDL statements can access and manipulate individuals partitions rather than entire tables or indexes. This is how partitioning can simplify the manageability of large database objects. Also, partitioning is entirely transparent to applications.
Partitioning is useful for many different types of applications, particularly applications that manage large volumes of data. OLTP systems often benefit from improvements in manageability and availability, while data warehousing systems benefit from performance and manageability.
Oracle includes datatypes to handle all the types of rich Internet content such as relational data, object-relational data, XML, text, audio, video, image, and spatial. These datatypes appear as native types in the database. They can all be queried using SQL. A single SQL statement can include data belonging to any or all of these datatypes.
XML, eXtensible Markup Language, is the standard way to identify and describe data on the Web. Oracle XML DB treats XML as a native datatype in the database. Oracle XML DB offers a number of easy ways to create XML documents from relational tables. The result of any SQL query can be automatically converted into an XML document. Oracle also includes a set of utilities, available in Java and C++, to simplify the task of creating XML documents.
Oracle includes five XML developer's kits, or XDKs. Each consists of a standards-based set of components, tools, and utilities. The XDKs are available for Java, C, C++, PL/SQL, and Java Beans.
The LOB datatypes BLOB, CLOB, NCLOB, and BFILE enable you to store and manipulate large blocks of unstructured data (such as text, graphic images, video clips, and sound waveforms) in binary or character format. They provide efficient, random, piece-wise access to the data.
Oracle Text indexes any document or textual content to add fast, accurate retrieval of information. Oracle Text allows text searches to be combined with regular database searches in a single SQL statement. The ability to find documents based on their textual content, metadata, or attributes, makes the Oracle Database the single point of integration for all data management.
The Oracle Text SQL API makes it simple and intuitive for application developers and DBAs to create and maintain Text indexes and run Text searches.
Oracle Ultra Search lets you index and search Web sites, database tables, files, mailing lists, Oracle Application Server Portals, and user-defined data sources. As such, you can use Oracle Ultra Search to build different kinds of search applications.
Oracle interMedia provides an array of services to develop and deploy traditional, Web, and wireless applications that include image, audio, and video in an integrated fashion. Multimedia content can be stored and managed directly in Oracle, or Oracle can store and index metadata together with external references that enable efficient access to media content stored outside the database.
Oracle includes built-in spatial features that let you store, index, and manage location content (assets, buildings, roads, land parcels, sales regions, and so on.) and query location relationships using the power of the database. The Oracle Spatial Option adds advanced spatial features such as linear reference support and coordinate systems.
Oracle includes security features that control how a database is accessed and used. For example, security mechanisms:
Prevent unauthorized database access
Prevent unauthorized access to schema objects
Audit user actions
Associated with each database user is a schema by the same name. By default, each database user creates and has access to all objects in the corresponding schema.
Database security can be classified into two categories: system security and data security.
System security includes the mechanisms that control the access and use of the database at the system level. For example, system security includes:
Valid user name/password combinations
The amount of disk space available to a user's schema objects
The resource limits for a user
System security mechanisms check whether a user is authorized to connect to the database, whether database auditing is active, and which system operations a user can perform.
Data security includes the mechanisms that control the access and use of the database at the schema object level. For example, data security includes:
Which users have access to a specific schema object and the specific types of actions allowed for each user on the schema object (for example, user SCOTT can issue SELECT and INSERT statements but not DELETE statements using the employees table)
The actions, if any, that are audited for each schema object
Data encryption to prevent unauthorized users from bypassing Oracle and accessing data
The Oracle database provides discretionary access control, which is a means of restricting access to information based on privileges. The appropriate privilege must be assigned to a user in order for that user to access a schema object. Appropriately privileged users can grant other users privileges at their discretion.
Oracle manages database security using several different facilities:
Authentication to validate the identity of the entities using your networks, databases, and applications
Authorization processes to limit access and actions, limits that are linked to user's identities and roles.
Access restrictions on objects, like tables or rows.
Security policies
Database auditing
Data must adhere to certain business rules, as determined by the database administrator or application developer. For example, assume that a business rule says that no row in the inventory table can contain a numeric value greater than nine in the sale_discount column. If an INSERT or UPDATE statement attempts to violate this integrity rule, then Oracle must undo the invalid statement and return an error to the application. Oracle provides integrity constraints and database triggers to manage data integrity rules.
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Note: Database triggers let you define and enforce integrity rules, but a database trigger is not the same as an integrity constraint. Among other things, a database trigger does not check data already loaded into a table. Therefore, it is strongly recommended that you use database triggers only when the integrity rule cannot be enforced by integrity constraints. |
An integrity constraint is a declarative way to define a business rule for a column of a table. An integrity constraint is a statement about table data that is always true and that follows these rules:
If an integrity constraint is created for a table and some existing table data does not satisfy the constraint, then the constraint cannot be enforced.
After a constraint is defined, if any of the results of a DML statement violate the integrity constraint, then the statement is rolled back, and an error is returned.
Integrity constraints are defined with a table and are stored as part of the table's definition in the data dictionary, so that all database applications adhere to the same set of rules. When a rule changes, it only needs be changed once at the database level and not many times for each application.
The following integrity constraints are supported by Oracle:
NOT NULL: Disallows nulls (empty entries) in a table's column.
UNIQUE KEY: Disallows duplicate values in a column or set of columns.
PRIMARY KEY: Disallows duplicate values and nulls in a column or set of columns.
FOREIGN KEY: 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.
CHECK: Disallows values that do not satisfy the logical expression of the constraint.
Key is used in the definitions of several types of integrity constraints. A key is the 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. Individual values in a key are called key values.
The different types of keys include:
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.
Unique key: The column or set of columns
