Release 2 (9.2)
Part Number A96524-01
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| Oracle9i Database Concepts Release 2 (9.2) Part Number A96524-01 |
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This chapter provides an overview of the Oracle server. The topics include:
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Note: This chapter contains information relating to both Oracle9i Standard Edition and Oracle9i Enterprise Edition. Some of the features and options documented in this chapter are available only if you have purchased the Oracle9i Enterprise Edition. See Oracle9i Database New Features for information about the differences between Oracle9i Standard Edition and Oracle9i 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.
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 logical structures of an Oracle database include schema objects, data blocks, extents, segments, and tablespaces.
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.
| See Also:
Chapter 10, "Schema Objects" for detailed information on these schema objects, and for information on other schema objects, such as dimensions, the sequence generator, synonyms, index-organized tables, domain indexes, clusters, and hash clusters |
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. Oracle stores each row of a database table containing data for less than 256 columns as one or more row pieces. 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.
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.
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.
You can partition indexes.
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 or not 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.
The logical storage structures, including data blocks, extents, and segments, enable Oracle to have fine-grained control of disk space use.
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 initialization parameter DB_BLOCK_SIZE. In addition, you can specify of 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. |
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Index segment |
Each index has an index segment that stores all of its data. For a partitioned index, each partition has an index segment. |
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Temporary segment |
Temporary segments are created by Oracle when a SQL statement needs a temporary work area to complete execution. When the statement finishes execution, the extents in the temporary segment are returned to the system for future use. |
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Rollback segment |
If you are operating in automatic undo management mode, then the database server manages undo space using tablespaces. Oracle Corporation recommends that you use "Automatic Undo Management" management. However, if you are operating in manual undo management mode, then one or more rollback segments for a database are created by the database administrator to temporarily store undo information. The information in a rollback segment is used during database recovery:
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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 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.
The relationship between databases, tablespaces, and datafiles (datafiles are described in the next section) is illustrated in Figure 1-1.
This figure illustrates the following:
SYSTEM tablespace has 2 megabit (Mb) storage capacity, and USERS tablespace has 4 Mb).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.
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:
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.
| See Also:
"Memory Structure and Processes Overview" for more information about Oracle's memory and process structures |
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.
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:
Like the redo log, Oracle lets the control file be multiplexed for protection of 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.
| See Also:
"Database Backup and Recovery Overview" for more information about the use of control files in database recovery |
The three utilities for moving a subset of an Oracle database from one database to another are Export, Import, and SQL*Loader.
The Export utility transfers data objects between Oracle databases, even if they reside on platforms with different hardware and software configurations. Export extracts the object definitions and table data from an Oracle database and stores them in an Oracle binary-format Export dump file typically located on disk or tape.
Such files can then be copied using file transfer protocol (FTP) or physically transported (in the case of tape) to a different site. They can be used with the Import utility to transfer data between databases that are on machines not connected through a network or as backups in addition to normal backup procedures.
When you run Export against an Oracle database, it extracts objects, such as tables, followed by their related objects, and then writes them to the Export dump file.
The Import utility inserts the data objects extracted from one Oracle database by the Export utility into another Oracle database. Export dump files can be read only by Import. Import reads the object definitions and table data that the Export utility extracted from an Oracle database.
The Export and Import utilities can also facilitate certain aspects of Oracle Advanced Replication functionality, such as offline instantiation.
Export dump files can be read only by the Oracle Import utility. If you need to read load data from ASCII fixed-format or delimited files, you can use the SQL*Loader utility. SQL*Loader loads data from external files into tables in an Oracle database. SQL*Loader accepts input data in a variety of formats, performs filtering (selectively loading records based on their data values), and loads data into multiple Oracle database tables during the same load session.
| See Also:
Oracle9i Database Utilities for detailed information about Export, Import, and SQL*Loader |
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:
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.
This section explains how Oracle adheres to industry accepted standards for data access languages, and how Oracle controls data consistency and data integrity. This section includes the following topics:
SQL (pronounced SEQUEL) is the programming language that defines and manipulates the database. SQL databases are relational databases, which means that data is stored in a set of simple relations.
All operations on the information in an Oracle database are performed using SQL statements. A SQL statement is a string of SQL text. A statement must be the equivalent of a complete SQL sentence, as in:
SELECT last_name, department_id FROM employees;
Only a complete SQL statement can run successfully. A sentence fragment, such as the following, generates an error indicating that more text is required:
SELECT last_name
A SQL statement can be thought of as a very simple, but powerful, computer program or instruction. SQL statements are divided into the following categories:
These statements create, alter, maintain, and drop schema objects. DDL statements also include statements that permit a user to grant other users the privileges to access the database and specific objects within the database.
These statements manipulate data. For example, querying, inserting, updating, and deleting rows of a table are all DML operations. The most common SQL statement is the SELECT statement, which retrieves data from the database. Locking a table or view and examining the execution plan of an SQL statement are also DML operations.
These statements manage the changes made by DML statements. They enable a user to group changes into logical transactions. Examples include COMMIT, ROLLBACK, and SAVEPOINT.
These statements let a user control the properties of the current session, including enabling and disabling roles and changing language settings. The two session control statements are ALTER SESSION and SET ROLE.
These statements change the properties of the Oracle server instance. The only system control statement is ALTER SYSTEM. It lets users change settings, such as the minimum number of shared servers, kill a session, and perform other tasks.
These statements incorporate DDL, DML, and transaction control statements in a procedural language program, such as those used with the Oracle precompilers. Examples include OPEN, CLOSE, FETCH, and EXECUTE.
See Also:
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Oracle object technology is a layer of abstraction built on Oracle's relational technology. New object types can be created from any built-in database types or any previously created object types, object references, and collection types. Metadata for user-defined types is stored in a schema available to SQL, PL/SQL, Java, and other published interfaces.
An object type differs from native SQL datatypes in that it is user-defined, and it specifies both the underlying persistent data (attributes) and the related behaviors (methods). Object types are abstractions of the real-world entities, for example, purchase orders.
Object types and related object-oriented features, such as variable-length arrays and nested tables, provide higher-level ways to organize and access data in the database. Underneath the object layer, data is still stored in columns and tables, but you can work with the data in terms of the real-world entities--customers and purchase orders, for example--that make the data meaningful. Instead of thinking in terms of columns and tables when you query the database, you can simply select a customer.
Internally, statements about objects are still basically statements about relational tables and columns, and you can continue to work with relational data types and store data in relational tables. But you have the option to take advantage of object-oriented features too. You can use object-oriented features while continuing to work with most of your relational data, or you can go over to an object-oriented approach entirely. For instance, you can define some object data types and store the objects in columns in relational tables. You can also create object views of existing relational data to represent and access this data according to an object model. Or you can store object data in object tables, where each row is an object.
In general, the object-type model is similar to the class mechanism found in C++ and Java. Like classes, objects make it easier to model complex, real-world business entities and logic, and the reusability of objects makes it possible to develop database applications faster and more efficiently. By natively supporting object types in the database, Oracle enables application developers to directly access the data structures used by their applications. No mapping layer is required between client-side objects and the relational database columns and tables that contain the data. Object abstraction and the encapsulation of object behaviors also make applications easier to understand and maintain.
PL/SQL is Oracle's procedural language extension to SQL. PL/SQL combines the ease and flexibility of SQL with the procedural functionality of a structured programming language, such as IF ... THEN, WHILE, and LOOP.
When designing a database application, consider the following advantages of using stored PL/SQL:
The following sections describe the PL/SQL program units that can be defined and stored centrally in a database.
Program units are stored procedures, functions, packages, triggers, and anonymous transactions.
Procedures and functions are sets of SQL and PL/SQL statements grouped together as a unit to solve a specific problem or to perform a set of related tasks. They are created and stored in compiled form in the database and can be run by a user or a database application.
Procedures and functions are identical, except that functions always return a single value to the user. Procedures do not return values.
Packages encapsulate and store related procedures, functions, variables, and other constructs together as a unit in the database. They offer increased functionality (for example, global package variables can be declared and used by any procedure in the package). They also improve performance (for example, all objects of the package are parsed, compiled, and loaded into memory once).
Database triggers are PL/SQL, Java, or C procedures that run implicitly whenever a table or view is modified or when some user actions or database system actions occur. Database triggers can be used in a variety of ways for managing your database. For example, they can automate data generation, audit data modifications, enforce complex integrity constraints, and customize complex security authorizations.
You can call autonomous transactions from within a PL/SQL block. When an autonomous PL/SQL block is entered, the transaction context of the caller is suspended. This operation ensures that SQL operations performed in this block (or other blocks called from it) have no dependence or effect on the state of the caller's transaction context.
Java is an object-oriented programming efficient for application-level programs. Java has key features that make it ideal for developing server applications. These features include the following:
XML, eXtensible Markup Language, is the standard way to identify and describe data on the Web. It is a human-readable, machine-understandable, general syntax for describing hierarchical data, applicable to a wide range of applications, databases, e-commerce, Java, web development, searching, and so on.
The Oracle server includes the Oracle XML DB, a set of built-in high-performance XML storage and retrieval technologies. The XML DB fully absorbs the W3C XML data model into the Oracle server and provides new standard access methods for navigating and querying XML. You get all the advantages of relational database technology and XML technology at the same time. Key aspects of the XML database include the following:
XMLType -- to store and manipulate XML. Multiple storage options (CLOB, decomposed object-relational) are available with XMLType, and DBAs can choose a storage that meets their requirements for fidelity to original, ease of query, ease of regeneration, and so on. With XMLType, you can perform SQL operations, such as queries and OLAP functions on XML data, as well as XML operations, such as XPath searches and XSL transformations, on SQL data. You can build regular SQL indexes or Oracle Text indexes on XMLType for high performance for a broad spectrum of applications.Complementing the XML Database is the Oracle XML Developer Kit, or XDK. XDK is a set of commonly used building blocks or utilities for development and runtime support. The Oracle XDK contains the basic building blocks for reading, manipulating, transforming, and viewing XML documents. To provide a broad variety of deployment options, the Oracle XDKs are available for Java, JavaBeans, C, C++, and PL/SQL. Oracle XDKs consist of XML Parsers, an XSLT Processor, XML Schema Processor, XML Class Generator, XML Transviewer Java Beans, XML SQL Utility, and XSQL Servlet.
Advanced Queuing (AQ) is the message queuing functionality of the Oracle database. With this functionality, message queuing operations can be performed similar to that of SQL operations from the Oracle database. Message queuing functionality enables asynchronous communication between applications and users on Oracle databases using queues. AQ offers enqueue, dequeue, propagation, and guaranteed delivery of messages, along with exception handling in case messages cannot be delivered. Message queuing takes advantage of XMLType for XML message payloads.
A transaction is a logical unit of work that comprises one or more SQL statements run by a single user. According to the ANSI/ISO SQL standard, with which Oracle is compatible, a transaction begins with the user's first executable SQL statement. A transaction ends when it is explicitly committed or rolled back by that user.
Consider a banking database. When a bank customer transfers money from a savings account to a checking account, the transaction can consist of three separate operations: decrease the savings account, increase the checking account, and record the transaction in the transaction journal.
Oracle must guarantee that all three SQL statements are performed to maintain the accounts in proper balance. When something prevents one of the statements in the transaction from running (such as a hardware failure), then the other statements of the transaction must be undone. This is called rolling back. If an error occurs in making any of the updates, then no updates are made.
Figure 1-2 illustrates the banking transaction example.
| See Also:
Oracle9i SQL Reference for information about Oracle's compliance with ANSI/ISO standards |
The changes made by the SQL statements that constitute a transaction can be either committed or rolled back. After a transaction is committed or rolled back, the next transaction begins with the next SQL statement.
Committing a transaction makes permanent the changes resulting from all SQL statements in the transaction. The changes made by the SQL statements of a transaction become visible to other user sessions' transactions that start only after the transaction is committed.
Rolling back a transaction retracts any of the changes resulting from the SQL statements in the transaction. After a transaction is rolled back, the affected data is left unchanged, as if the SQL statements in the transaction were never run.
Savepoints divide a long transaction with many SQL statements into smaller parts. With savepoints, you can arbitrarily mark your work at any point within a long transaction. This gives you the option of later rolling back all work performed from the current point in the transaction to a declared savepoint within the transaction. For example, you can use savepoints throughout a long complex series of updates, so if you make an error, you do not need to resubmit every statement.
Transactions let users guarantee consistent changes to data, as long as the SQL statements within a transaction are grouped logically. A transaction should consist of all of the necessary parts for one logical unit of work--no more and no less. Data in all referenced tables are in a consistent state before the transaction begins and after it ends. Transactions should consist of only the SQL statements that make one consistent change to the data.
For example, recall the banking example. A transfer of funds between two accounts (the transaction) should include increasing one account (one SQL statement), decreasing another account (one SQL statement), and recording the transaction in the journal (one SQL statement). All actions should either fail or succeed together; the credit should not be committed without the debit. Other nonrelated actions, such as a new deposit to one account, should not be included in the transfer of funds transaction. Such statements should be in other transactions.
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 roll back the invalid statement and return an error to the application. Oracle provides integrity constraints and database triggers to manage data integrity rules.
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 a table's data that is always true and that follows these rules:
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 constraint. A primary key's values uniquely identify the rows in a table. Only one primary key can be defined for each table.UNIQUE constraint.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. Through SQL*Plus, you can:
An Oracle 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.
The architectural features discussed in this section enable the Oracle server to support:
Figure 1-3 shows a typical variation of the Oracle server memory and process structures.
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Note: In a UNIX environment, most Oracle processes are part of one master Oracle process, rather than being individual processes. On Windows NT, all processes consist of at least one thread. A thread is an individual execution within a process. Threads enable concurrent operations within a process so that a process can run different parts of its program simultaneously on different processors. A thread is the most fundamental component that can be scheduled on Windows NT. In UNIX documentation, such as this book, whenever the word "process" is mentioned, it is considered a "thread" on Windows NT. |
An Oracle server consists of an Oracle database and an Oracle server 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 take advantage of such architecture by running multiple instances that share a single physical database. In most applications, Real Application Clusters enable access to a single database by users on multiple machines with increased performance.
Real Application Clusters are inherently high availability systems. The clusters that are typical of Real Application Clusters environments can provide continuous service for both planned and unplanned outages.
An Oracle 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 server 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.
The large pool is an optional area that provides large memory allocations for Oracle backup and restore operations, I/O server processes, and session memory for the shared server and Oracle XA (used where transactions interact with more than one database).
A cursor is a handle (a name or pointer) for the memory associated with a specific statement. (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.
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 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 a Pro*C/C++ 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. The different types of Oracle processes and their specific functions are discussed in the following sections.
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.
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.
Each Oracle instance can use several background processes. The names of these processes are DBWn, LGWR, CKPT, SMON, PMON, ARCn, RECO, Jnnn, Dnnn, LMS, and QMNn.
The database writer writes modified blocks from the database buffer cache to the datafiles. Although one database writer process (DBW0) is sufficient for most systems, you can configure additional processes (DBW1 through DBW9 and DBWa through DBWj) to improve write performance for a system that modifies data heavily. The initialization parameter DB_WRITER_PROCESSES specifies the number of DBWn processes.
Because Oracle uses write-ahead logging, DBWn does not need to write blocks when a transaction commits. Instead, DBWn is designed to perform batched writes with high efficiency. In the most common case, DBWn writes only when more data needs to be read into the SGA and too few database buffers are free. The least recently used data is written to the datafiles first. DBWn also performs writes for other functions, such as checkpointing.
The log writer writes redo log entries to disk. Redo log entries are generated in the redo log buffer of the SGA, and LGWR writes the redo log entries sequentially into an online redo log. If the database has a multiplexed redo log, then LGWR writes the redo log entries to a group of online redo log files.
At specific times, all modified database buffers in the SGA are written to the datafiles by DBWn. This event is called a checkpoint. The checkpoint process is responsible for signaling DBWn at checkpoints and updating all the datafiles and control files of the database to indicate the most recent checkpoint.
The system monitor performs recovery when a failed instance starts up again. With Real Application Clusters, the SMON process of one instance can perform instance recovery for other instances that have failed. SMON also cleans up temporary segments that are no longer in use and recovers terminated transactions skipped during recovery because of file-read or offline errors. These transactions are eventually recovered by SMON when the tablespace or file is brought back online. SMON also coalesces free extents in the dictionary managed tablespaces to make free space contiguous and easier to allocate.
The process monitor performs process recovery when a user process fails. PMON is responsible for cleaning up the cache and freeing resources that the process was using. PMON also checks on dispatcher and server processes and restarts them if they have failed.
The archiver copies the online redo log files to archival storage after a log switch has occurred. Although a single ARCn process (ARC0) is sufficient for most systems, you can specify up to 10 ARCn processes by using the dynamic initialization parameter LOG_ARCHIVE_MAX_PROCESSES. If the workload becomes too great for the current number of ARCn processes, then LGWR automatically starts another ARCn process up to the maximum of 10 processes. ARCn is active only when a database is in ARCHIVELOG mode and automatic archiving is enabled.
The recoverer is used to resolve distributed transactions that are pending due to a network or system failure in a distributed database. At timed intervals, the local RECO attempts to connect to remote databases and automatically complete the commit or rollback of the local portion of any pending distributed transactions.
Job queue processes are used for batch processing. Job queue processes are managed dynamically. This enables job queue clients to use more job queue processes when required. The resources used by the new processes are released when they are idle.
See Also:
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Dispatchers are optional background processes, present only when a shared server configuration is used. At least one dispatcher process is created for every communication protocol in use (D000, . . ., Dnnn). Each dispatcher process is responsible for routing requests from connected user processes to available shared server processes and returning the responses back to the appropriate user processes.
The Lock Manager Server process (LMS) is used for inter-instance locking in Real Application Clusters.
| See Also:
"Real Application Clusters: Multiple Instance Systems" for more information about the configuration of the lock process |
Queue monitors are optional background processes that monitor the message queues for Oracle Advanced Queuing. You can configure up to 10 queue monitor processes.
The program interface is the mechanism by which a user process communicates with a server process. It serves as a method of standard communication between any client tool or application (such as Oracle Forms) and Oracle software. Its functions are to:
If the user and server processes are on different computers of a network, or if user processes connect to shared server processes through dispatcher processes, then the user process and server process communicate using Oracle Net Services. Dispatchers are optional background processes, present only in the shared server configuration.
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. In a networked environment, an Oracle database server communicates with client workstations and other Oracle database servers using Oracle Net Services software.
Oracle Net Services supports communications on all major network protocols, ranging from those supported by PC LANs to those used by the largest of mainframe computer systems.
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.
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).
| See Also:
Chapter 8, "Process Architecture" for more information about Oracle configuration |
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 the front-end database application, accessed by a user through the keyboard, display, and pointing device, such as a mouse. The client has no data access responsibilities. 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:
This architecture enables use of an application server to:
The identity of the client is maintained throughout all tiers of the connection.
A distributed database is a network of databases managed by multiple database servers that are used together. They are not usually seen as a single logical database. The data of all databases in the distributed database can be simultaneously accessed and modified. The primary benefit of a distributed database is that the data of physically separate databases can be logically combined and potentially made accessible to all users on a network.
Each computer that manages a database in the distributed database is called a node. The database to which a user is directly connected is called the local database. Any additional databases accessed by this user are called remote databases. When a local database accesses a remote database for information, the local database is a client of the remote server. This is an example of client/server architecture.
A database link describes a path from one database to another. Database links are implicitly used when a reference is made to a global object name in a distributed database.
While a distributed database enables increased access to a large amount of data across a network, it must also hide the location of the data and the complexity of accessing it across the network. The distributed database management system must also preserve the advantages of administrating each local database as though it were not distributed.
| See Also :
Oracle9i Database Administrator's Guide for more information about distributed databases |
Location transparency occurs when the physical location of data is transparent to the applications and users of a database system. Several Oracle features, such as views, procedures, and synonyms, can provide location transparency. For example, a view that joins table data from several databases provides location transparency because the user of the view does not need to know from where the data originates.
Site autonomy means that each database participating in a distributed database is administered separately and independently from the other databases, as though each database were a non-networked database. Although each database can work with others, they are distinct, separate systems that are cared for individually.
The Oracle distributed database architecture supports all DML operations, including queries, inserts, updates, and deletes of remote table data. To access remote data, you make reference to the remote object's global object name. No coding or complex syntax is required to access remote data.
For example, to query a table named employees in the remote database named sales, reference the table's global object name:
SELECT * FROM employees@sales;
Oracle provides the same assurance of data consistency in a distributed environment as in a nondistributed environment. Oracle provides this assurance using the transaction model and a two-phase commit mechanism.
As in nondistributed systems, transactions should be carefully planned to include a logical set of SQL statements that should all succeed or fail as a unit. Oracle's two-phase commit mechanism guarantees that no matter what type of system or network failure occurs, a distributed transaction either commits on all involved nodes or rolls back on all involved nodes to maintain data consistency across the global distributed database.
Replication is the process of copying and maintaining database objects, such as tables, in multiple databases that make up a distributed database system. Changes applied at one site are captured and stored locally before being forwarded and applied at each of the remote locations. Oracle replication is a fully integrated feature of the Oracle server. It is not a separate server.
Replication uses distributed database technology to share data between multiple sites, but a replicated database and a distributed database are not the same. In a distributed database, data is available at many locations, but a particular table resides at only one location. For example, the employees table can reside at only the db1 database in a distributed database system that also includes the db2 and db3 databases. Replication means that the same data is available at multiple locations. For example, the employees table can be available at db1, db2, and db3.
Distributed database systems often locally replicate remote tables that are frequently queried by local users. By having copies of heavily accessed data on several nodes, the distributed database does not need to send information across a network repeatedly, thus helping to maximize the performance of the database application.
Data can be replicated using materialized views.
Oracle supports materialized views that are hierarchical and updatable. Multitier replication provides increased flexibility of design for a distributed application. Using multitier materialized views, applications can manage multilevel data subsets with no direct connection between levels.
An updatable materialized view lets you insert, update, and delete rows in the materialized view and propagate the changes to the target master table. Synchronous and asynchronous replication is supported.
Figure 1-4 shows an example of multitier architecture, diagrammed as an inverted tree structure. Changes are propagated up and down along the branches connecting the outermost materialized views with the master (the root).
In Oracle9i conflict resolution routines are defined at the top level, the master site, and are pulled into the updatable materialized view site when needed. This makes it possible to have multitier materialized views. Existing system-defined conflict resolution methods are supported.
In addition, users can write their own conflict resolution routines. A user-defined conflict resolution method is a PL/SQL function that returns either true or false. True indicates that the method was able to successfully resolve all conflicting modifications for a column group.
| See Also:
Oracle9i Replication and Oracle9i SQL Reference for more information about creating and managing multitier materialized views |
Oracle Streams enables the sharing of data and events in a data stream, either within a database or from one database to another. The stream routes specified information to specified destinations. Oracle Streams provides the capabilities needed to build and operate distributed enterprises and applications, data warehouses, and high availability solutions. You can use all the capabilities of Oracle Streams at the same time. If your needs change, you can implement a new capability of Streams without sacrificing existing capabilities.
Using Oracle Streams, you control what information is put into a stream, how the stream flows or is routed from database to database, what happens to events in the stream as they flow into each database, and how the stream terminates. By configuring specific capabilities of Streams, you can address specific requirements. Based on your specifications, Streams can capture and manage events in the database automatically, including, but not limited to, DML changes and DDL changes. You can also put user-defined events into a stream. Then, Streams can propagate the information to other databases or applications automatically. Again, based on your specifications, Streams can apply events at a destination database.
You can use Streams to:
You can configure a background capture process to capture changes made to tables, schemas, or the entire database. A capture process captures changes from the redo log and formats each captured change into a logical change record (LCR). The database where changes are generated in the redo log is called the source database.
A capture process enqueues LCR events into a queue that you specify. The queue can then share the LCR events within the same database or with other databases.
You can also enqueue user events into a queue explicitly with a user application. These explicitly enqueued events can be LCRs or user messages.
A background apply process can dequeue events from a queue. You can also dequeue events explicitly with a user application.
You can configure an apply process to apply all of the events in a queue or only the events that you specify. You can also configure an apply process to call your own PL/SQL subprograms to process events.
The database where LCR events are applied and other types of events are processed is called the destination database. In some configurations, the source database and the destination database may be the same.
Other capabilities of Streams include the following:
Oracle Advanced Queuing provides an infrastructure for distributed applications to communicate asynchronously using messages. Oracle Advanced Queuing stores messages in queues for deferred retrieval and processing by the Oracle server. This provides a reliable and efficient queuing system without additional software such as transaction processing monitors or message-oriented middleware.
Messages pass between clients and servers, as well as between processes on different servers. An effective messaging system implements content-based routing, subscription, and querying.
A messaging system can be classified into one of two types: