|Oracle® Database Concepts
11g Release 2 (11.2)
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The means by which data is retrieved from a database. For example, a query using an index and a query using a full table scan use different access paths.
Automatic Database Diagnostic Monitor. An Oracle Database infrastructure that enables a database to diagnose its own performance and determine how identified problems could be resolved.
Automatic Diagnostic Repository. A a file-based hierarchical data store for managing diagnostic information, including network tracing and logging.
A file that provides a chronological log of database messages and errors. The alert log is stored in the ADR.
A member of the online redo log that has been archived by Oracle Database. The archived redo log files can be applied to a database backup in media recovery.
A mode of the database that enables the archiving of the online redo log.
A mode of the database in which it automatically manages undo space in a dedicated undo tablespace.
See also manual undo management mode.
Automatic Workload Repository (AWR). A built-in repository in every Oracle database. Oracle Database periodically makes a snapshot of its vital statistics and workload information and stores them in AWR.
An index organized like an upside-down tree. A B-tree index has two types of blocks: branch blocks for searching and leaf blocks that store values. The leaf blocks contain every indexed data value and a corresponding rowid used to locate the actual row. The "B" stands for "balanced" because all leaf blocks automatically stay at the same depth.
A process that consolidates functions that would otherwise be handled by multiple Oracle programs running for each client process. The background processes asynchronously perform I/O and monitor other Oracle processes.
A placeholder in a SQL statement that must be replaced with a valid value or value address for the statement to execute successfully. By using bind variables, you can write a SQL statement that accepts inputs or parameters at run time. The following example shows a query that uses
v_empid as a bind variable:
SELECT * FROM employees WHERE employee_id = :v_empid;
A database index in which the database stores a bitmap for each index key instead of a list of rowids.
An operation that merges bitmaps retrieved from bitmap index scans. For example, if the
DOB columns have bitmap indexes, then the database may use a bitmap merge if the query predicate is
WHERE gender='F' AND DOB > 1966.
A main memory address in the database buffer cache. A buffer caches currently and recently used data blocks read from disk. When a new block is needed, the database can replace an old data block with a new one.
The ratio of distinct values to the number of table rows. A column with only two distinct values in a million-row table would have low cardinality.
1. A data structure that marks the checkpoint position, which is the SCN in the redo thread where instance recovery must begin. Checkpoints are recorded in the control file and each data file header, and are a crucial element of recovery.
In client/server architecture, the front-end database application that interacts with a user. The client portion has no data access responsibilities.
A process that executes the application or Oracle tool code. When users run client applications such as SQL*Plus, the operating system creates client processes to run the applications.
See also Oracle process.
Software architecture based on a separation of processing between two CPUs, one acting as the client in the transaction, requesting and receiving services, and the other as the server that provides services in a transaction.
Vertical space in a table that represents a domain of data. A table definition includes a table name and set of columns. Each column has a name and data type.
Action that ends a database transaction and makes permanent all changes performed in the transaction.
Simultaneous access of the same data by many users. A multiuser database management system must provide adequate concurrency controls so that data cannot be updated or changed improperly, compromising data integrity.
See also data consistency.
See also session.
A whole database backup that you can open with the
RESETLOGS option without performing media recovery. In other words, the backup does not require the application of redo to be made consistent.
See also inconsistent backup.
A binary file that records the physical structure of a database and contains the names and locations of redo log files, the time stamp of the database creation, the current log sequence number, checkpoint information, and so on.
An organization of measures with identical dimensions and other shared characteristics. The edges of the cube contain the dimension members, whereas the body of the cube contains the data values.
Organized collection of data treated as a unit. The purpose of a database is to store and retrieve related information. Every Oracle database instance accesses only one database in its lifetime.
Smallest logical unit of data storage in Oracle Database. Other names for data blocks include Oracle blocks or pages. One data block corresponds to a specific number of bytes of physical space on disk.
A consistent view of the data by each user in a multiuser database.
See also concurrency.
A read-only collection of database tables and views containing reference information about the database, its structures, and its users.
A memory area in the shared pool that holds data dictionary information. The data dictionary cache is also known as the row cache because it holds data as rows instead of buffers, which hold entire data blocks.
A predefined view of tables or other views in the data dictionary. Data dictionary views begin with the prefix
A physical file on disk that was created by Oracle Database and contains the data for a database. The data files can be located either in an operating system file system or Oracle ASM disk group.
The automated search of large stores of data for patterns and trends that transcend simple analysis.
An Oracle Database infrastructure that automatically diagnoses persistent data failures, presents repair options to the user, and executes repairs at the user's request.
See also extent.
In SQL, a fixed set of properties associated with a column value or constant. Examples include
NUMBER. Oracle Database treats values of different data types differently.
Data definition language. Includes statements such as
CREATE TABLE or
ALTER INDEX that define or change a data structure.
See also shared server.
A constraint that permits a
SET CONSTRAINT statement to defer constraint checking until after the transaction is committed. A deferrable constraint enables you to disable the constraint temporarily while making changes that might violate the constraint.
A relational table that stores all or part of the values for a dimension in a star or snowflake schema. Dimension tables typically contain columns for the dimension keys, levels, and attributes.
A database object that specifies an alias for a directory on the server file system where external binary file LOBs (BFILEs) and external table data are located. All directory objects are created in a single namespace and are not owned by an individual schema.
Optional background process present only when a shared server configuration is used. Each dispatcher process is responsible for routing requests from connected client processes to available shared server processes and returning the responses.
A set of databases in a distributed system that can appear to applications as a single data source.
The operations that occurs when an application distributes its tasks among different computers in a network.
A transaction that includes statements that, individually or as a group, update data on nodes of a distributed database. Oracle Database ensures the integrity of data in distributed transactions using the two-phase commit mechanism.
A private environment in which you can redefine database objects. Edition-based redefinition enables you to upgrade an application's database objects while the application is in use, thus minimizing or eliminating down time.
Extraction, transformation, and loading (ETL). The process of extracting data from source systems and bringing it into a data warehouse.
The combination of steps used by the database to execute a SQL statement. Each step either retrieves rows of data physically from the database or prepares them for the user issuing the statement. You can override execution plans by using hints.
A combination of one or more values, operators, and SQL functions that evaluates to a value. For example, the expression
2*2 evaluates to
4. In general, expressions assume the data type of their components.
Multiple contiguous data blocks allocated for storing a specific type of information. A segment is made up of one or more extents.
See also data block.
A read-only table whose metadata is stored in the database but whose data in stored in files outside the database. The database uses the metadata describing external tables to expose their data as if they were relational tables.
A full index scan in which the database reads the index blocks in no particular order. The database accesses the data in the index itself, without accessing the table.
An integrity constraint that requires each value in a column or set of columns to match a value in the unique or primary key for a related table. Integrity constraints for foreign keys define actions dictating database behavior if referenced data is altered.
A scan of an index in which the database reads the entire index in order.
A scan of table data in which the database sequentially reads all rows from a table and filters out those that do not meet the selection criteria. All data blocks under the high water mark are scanned.
A schema object, similar to a procedure, that always returns a single value.
A computing architecture that coordinates large numbers of servers and storage to act as a single large computer.
The software that provides the infrastructure for an enterprise grid architecture. In a cluster, this software includes Oracle Clusterware and Oracle ASM. For a standalone server, this software includes Oracle Restart and Oracle ASM. Oracle Database 11g Release 2 (11.2) combines these infrastructure products into one software installation called the grid infrastructure home.
The steps performed by the database to build a new executable version of application code. The database must perform a hard parse instead of a soft parse if the parsed representation of a submitted statement does not exist in the shared pool.
A function that operates on an arbitrary-length input value and returns a fixed-length hash value.
A mathematical technique in which an infinite set of input values is mapped to a finite set of output values, called hash values. Hashing is useful for rapid lookups of data in a hash table.
A join in which the database uses the smaller of two tables or data sources to build a hash table in memory. The database scans the larger table, probing the hash table for the addresses of the matching rows in the smaller table.
An in-memory data structure that associates join keys with rows in a hash join. For example, in a join of the
departments tables, the join key might be the department ID. A hash function uses the join key to generate a hash value. This hash value is an index in an array, which is the hash table.
A table in which the data rows are stored in no particular order on disk. By default,
CREATE TABLE creates a heap-organized table.
The boundary between used and unused space in a segment.
A component of a DML statement that retrieves data without a subquery. An
MERGE statement that does not explicitly include a
SELECT statement uses an implicit query to retrieve the rows to be modified.
A measure of the row order in relation to an indexed value such as last name. The more order that exists in row storage for this value, the lower the clustering factor.
A configuration parameter such as
SGA_TARGET that affects the operation of a database instance. Settings for initialization parameters are stored in a text-based initialization parameter file or binary server parameter file.
The combination of the system global area (SGA) and background processes. An instance is associated with one and only one database. In an Oracle Real Application Clusters configuration, multiple instances access a single database simultaneously.
The termination of a database instance because of a hardware failure, Oracle internal error, or
SHUTDOWN ABORT statement.
The automatic application of redo log records to uncommitted data blocks when an instance is restarted after a failure.
See data integrity.
Declarative method of defining a rule for a column. The integrity constraints enforce business rules and prevent the entry of invalid information into tables.
A condition that compares two columns, each from a different table, in a join. The database combines pairs of rows, each containing one row from each table, for which the join condition evaluates to
Column or set of columns included in the definition of certain types of integrity constraints.
A low-level serialization control mechanism used to protect shared data structures in the SGA from simultaneous access.
An area of memory in the shared pool. This cache includes the shared SQL areas, private SQL areas (in a shared server configuration), PL/SQL procedures and packages, and control structures such as locks and library cache handles.
A process that listens for incoming client connection requests and manages network traffic to the database.
A database mechanism that prevents destructive interaction between transactions accessing a shared resource such as a table, row, or system object not visible to users. The main categories of locks are DML locks, DDL locks, and latches and internal locks.
A number that uniquely identifies a set of redo records in a redo log file. When the database fills one online redo log file and switches to a different one, the database automatically assigns the new file a log sequence number.
The point at which the log writer (LGWR) stops writing to the active redo log file and switches to the next available redo log file. LGWR switches when either the active redo log file is filled with redo records or a switch is manually initiated.
The background process responsible for redo log buffer management—writing the redo log buffer to the online redo log. LGWR writes all redo entries that have been copied into the buffer since the last time it wrote.
A table containing a code column and an associated value column. For example, a job code corresponds to a job name. In contrast to a master table in a pair of master-detail tables, a lookup table is not the means to obtain a detailed result set, such as a list of employees. Rather, a user queries a table such as
employees for an employee list and then joins the result set to the lookup table.
A data integrity problem in which one writer of data overwrites the changes of a different writer modifying the same data.
A mode of the database in which undo blocks are stored in user-managed undo segments. In automatic undo management mode, undo blocks are stored in a system-managed, dedicated undo tablespaces.
A detail table has a foreign key relationship with a master table. For example, the
employees detail table has a foreign key to the
departments master table. Unlike a lookup table, a master table is typically queried and then joined to the detail table. For example, a user may query a department in the
departments table and then use this result to find the employees in this department.
See also view.
A schema object that abstracts a real-world entity such as a purchase order. Attributes model the structure of the entity, whereas methods implement operations an application can perform on the entity.
Online Analytical Processing. OLAP is characterized by dynamic, dimensional analysis of historical data.
Online Transaction Processing. OLTP systems are optimized for fast and reliable transaction handling. Compared to data warehouse systems, most OLTP interactions involve a relatively small number of rows, but a larger group of tables.
The set of two or more online redo log files that record all changes made to Oracle Database data files and control file. When a change is made to the database, Oracle Database generates a redo record in the redo buffer. log writer (LGWR) writes the contents of the redo buffer to the online redo log.
2. In SQL, an operator manipulates data items called operands or arguments and returns a result. The SQL operators are represented by special characters or by keywords. For example, the multiplication operator is represented by an asterisk (
Built-in database software that determines the most efficient way to execute a SQL statement by considering factors related to the objects referenced and the conditions specified in the statement.
See Oracle ASM.
Oracle Automatic Storage Management (Oracle ASM). A volume manager and a file system for database files. Oracle ASM is Oracle's recommended storage management solution, providing an alternative to conventional volume managers, file systems, and raw devices.
One or more Oracle ASM disks managed as a logical unit. I/O to a disk group is automatically spread across all the disks in the group.
A set of components that enables servers to operate together as if they were one server. Oracle Clusterware is a requirement for using Oracle RAC and it is the only clusterware that you need for platforms on which Oracle RAC operates.
A system management tool that provides centralized management of an Oracle database environment.
Oracle Real Application Clusters. Option that allows multiple concurrent database instances to share a single physical database.
See Oracle RAC.
An external interface that allows global transactions to be coordinated by a transaction manager other than Oracle Database.
Program global area. A memory buffer that contains data and control information for a server process.
See also SGA.
Procedural Language/SQL. The Oracle Database procedural language extension to SQL. PL/SQL enables you to mix SQL statements with programmatic constructs such as procedures, functions, and packages.
A programming tool that enables you to embed SQL statements in a high-level source program written in a language such as C, C++, or COBOL.
The column or set of columns that uniquely identifies a row in a table. Only one primary key can be defined for each table.
An integrity constraint that disallows duplicate values and nulls in a column or set of columns.
An area in memory that holds a parsed statement and other information for processing. The private SQL area contains data such as bind variable values, query execution state information, and query execution work areas.
The right to run a particular type of SQL statement, or the right to access an object that belongs to another user, run a PL/SQL package, and so on. The types of privileges are defined by Oracle Database.
A schema object that consists of a set of SQL statements and other PL/SQL constructs, grouped together, stored in the database, and run as a unit to solve a specific problem or perform a set of related tasks.
A mechanism in an operating system that can run a series of steps. By dividing the work of Oracle Database and database applications into several processes, multiple users and applications can connect to a single database instance simultaneously.
An operation that retrieves data from tables or views. For example,
SELECT * FROM employees is a query.
The execution plan used to execute a query.
A consistent view of data seen by a user. For example, in statement-level read consistency the set of data seen by a SQL statement remains constant throughout statement execution.
Memory structure in the SGA that stores redo entries—a log of changes made to the database. The database writes the redo entries stored in the redo log buffers to an online redo log file, which is used if instance recovery is necessary.
The redo generated by a database instance.
A rule defined on a key in one table that guarantees that the values in that key match the values in a key in a related table (the referenced value).
Recovery Manager. An Oracle Database utility that backs up, restores, and recovers Oracle databases.
A set of privileges that can be granted to database users or to other roles.
A situation in which Oracle Database must store a row in a series or chain of blocks because it is too large to fit into a single block.
A situation in which Oracle Database moves a row from one data block to another data block because the row grows too large to fit in the original block.
A named collection of database objects, including logical structures such as tables and indexes. A schema has the name of the database user who owns it.
A logical structure of data stored in a schema. Examples of schema objects are tables, indexes, sequences, and database links.
System Change Number. A database ordering primitive. The value of an SCN is the logical point in time at which changes are made to a database.
In a query, the measure of how many rows from a row set pass a predicate test, for example,
WHERE last_name = 'Smith'. A selectivity of 0.0 means no rows, whereas a value of 1.0 means all rows. A predicate becomes more selective as the value approaches 0.0 and less selective (or more unselective) as the value approaches 1.0.
A schema object that generates a serial list of unique numbers for table columns.
In a client/server architecture, the computer that runs Oracle software and handles the functions required for concurrent, shared data access. The server receives and processes the SQL and PL/SQL statements that originate from client applications.
A server-side binary file containing initialization parameter settings that is read and written to by the database.
System global area. A group of shared memory structures that contain data and control information for one Oracle database instance.
Portion of the SGA that contains shared memory constructs such as shared SQL areas.
A database configuration that enables multiple client processes to share a small number of server processes.
See also dedicated server.
The reuse of existing code when the parsed representation of a submitted SQL statement exists in the shared pool and can be shared.
See also hard parse.
Structured Query Language. A nonprocedural language to access a relational database. Users describe in SQL what they want done, and the SQL language compiler automatically generates a procedure to navigate the database and perform the task. Oracle SQL includes many extensions to the ANSI/ISO standard SQL language.
Oracle tool used to run SQL statements against Oracle Database.
An independent copy of a production database that you can use for disaster protection in a high availability environment.
A database operation in which the effects of an unsuccessful SQL statement are rolled back because the statement caused an error during execution.
A named PL/SQL block or Java program that Oracle Database stores in the database. Applications can call stored procedures by name.
A query nested within another SQL statement. Unlike implicit queries, subqueries use a
SELECT statement to retrieve data.
An alias for a schema object. You can use synonyms to provide data independence and location transparency.
A schema object that contains data from one or more tables, all of which have one or more columns in common. In table clusters, the database stores together all the rows from all tables that share the same cluster key.
A database storage unit that groups related logical structures together. The database data files are stored in tablespaces.
A file that belongs to a temporary tablespace. The temp files in temporary tablespaces cannot contain permanent database objects.
A segment created by Oracle Database when a SQL statement needs a temporary database area to complete execution.
Logical unit of work that contains one or more SQL statements. All statements in a transaction commit or roll back together. The use of transactions is one of the most important ways that a database management system differs from a file system.
An identifier is unique to a transaction and represents the undo segment number, slot, and sequence number.
A phase of instance recovery in which uncommitted transactions are rolled back.
The data structure within an undo segment that holds the transaction identifiers of the transactions using the undo segment.
A PL/SQL or Java procedure that fires when a table or view is modified or when specific user or database actions occur. Procedures are explicitly run, whereas triggers are implicitly run.
User global area. Session memory that stores session variables, such as logon information, and can also contain the OLAP pool.
Records of the actions of transactions, primarily before they are committed. The database can use undo data to logically reverse the effect of SQL statements. Undo data is stored in undo segments.
The name by which a user is known to Oracle Database and to other users. Every user name is associated with a password, and both must be entered to connect to Oracle Database.
See client process.