| Oracle® Database Concepts 11g Release 2 (11.2) Part Number E10713-02 |
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| Oracle® Database Concepts 11g Release 2 (11.2) Part Number E10713-02 |
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View PDF |
access path
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.
ADDM
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.
ADR
Automatic Diagnostic Repository. A a file-based hierarchical data store for managing diagnostic information, including network tracing and logging.
alert log
A file that provides a chronological log of database messages and errors. The alert log is stored in the ADR.
archived redo log file
A member of the online redo log that has been archived by Oracle Database.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.
automatic undo management mode
A mode of the database in which it automatically manages undo space in a dedicated undo tablespace.
See also manual undo management mode.
AWR
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.
background process
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.
See also instance; Oracle process.
bind variable
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;
bitmap index
A database index in which the database stores a bitmap for each index key instead of a list of rowids.
bitmap merge
An operation that merges bitmaps retrieved from bitmap index scans. For example, if the gender and DOB columns have bitmap indexes, then the database may use a bitmap merge if the query predicate is WHERE gender='F' AND DOB > 1966.
block header
A part of a data block that includes information about the type of block, the address of the block, and sometimes transaction information.
block overhead
Space in a data block that stores metadata required for managing the block. The overhead includes the block header, table directory, and row directory.
buffer
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.
cache recovery
The phase of instance recovery where Oracle Database applies all committed and uncommitted changes in the online redo log files to the affected data blocks.
checkpoint
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.
2. The writing of dirty data blocks in the database buffer cache to disk. The database writer (DBWn) process writes blocks to disk to synchronize the buffer cache with the data files.
client
In client/server architecture, the front-end database application that interacts with a user. The client portion has no data access responsibilities.
client process
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.
client/server architecture
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.
column
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.
commit
Action that ends a database transaction and makes permanent all changes performed in the transaction.
concurrency
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.
condition
The combination of one or more expressions and logical operators in a SQL statement that returns a value of TRUE, FALSE, or UNKNOWN. For example, the condition 1=1 always evaluates to TRUE.
connection
Communication pathway between a client process and an Oracle database instance.
See also session.
connection pooling
A resource utilization and user scalability feature that maximizes the number of sessions over a limited number of protocol connections to a shared server.
consistent backup
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.
context
A set of application-defined attributes that validates and secures an application. The SQL statement CREATE CONTEXT creates namespaces for contexts.
control file
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.
cube
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.
database
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.
database buffer cache
The portion of the system global area (SGA) that holds copies of data blocks. All client processes concurrently connected to the instance share access to the buffer cache.
database link
In a schema object, a schema object in one database that enables users to access objects on a different database.
database writer (DBWn)
A background process that writes buffers in the database buffer cache to data files.
data block
Smallest logical unit of data storage in Oracle Database. Other names for data blocks include logical blocks, Oracle blocks, or pages. One data block corresponds to a specific number of bytes of physical space on disk.
data consistency
A consistent view of the data by each user in a multiuser database.
See also concurrency.
data dictionary
A read-only collection of database tables and views containing reference information about the database, its structures, and its users.
data dictionary cache
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.
data dictionary view
A predefined view of tables or other views in the data dictionary. Data dictionary views begin with the prefix DBA_, ALL_, or USER_.
data file
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.
data integrity
Business rules that dictate the standards for acceptable data. These rules are applied to a database by using integrity constraints and triggers to prevent invalid data entry.
data mining
The automated search of large stores of data for patterns and trends that transcend simple analysis.
Data Recovery Advisor
An Oracle Database infrastructure that automatically diagnoses persistent data failures, presents repair options to the user, and executes repairs at the user's request.
data segment
The segment containing the data for a nonclustered table, table partition, or table cluster.
See also extent.
data type
In SQL, a fixed set of properties associated with a column value or constant. Examples include VARCHAR2 and NUMBER. Oracle Database treats values of different data types differently.
DDL
Data definition language. Includes statements such as CREATE TABLE or ALTER INDEX that define or change a data structure.
deadlock
A situation in which two or more users are waiting for data locked by each other. Such deadlocks are rare in Oracle Database.
dedicated server
A database configuration in which a server process handles requests for a single client process.
See also shared server.
deferrable constraint
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.
dimension
A structure that categorizes data to enable users to answer business questions. Commonly used dimensions are customers, products, and time.
dimension table
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.
directory object
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.
dispatcher process (Dnnn)
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.
distributed database
A set of databases in a distributed system that can appear to applications as a single data source.
distributed processing
The operations that occurs when an application distributes its tasks among different computers in a network.
distributed transaction
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.
ETL
Extraction, transformation, and loading (ETL). The process of extracting data from source systems and bringing it into a data warehouse.
execution plan
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.
expression
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.
extent
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.
external table
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.
fact table
A table in a star schema of a data warehouse that contains factual data. A fact table typically has two types of columns: those that contain facts and those that are foreign keys to dimension tables.
fast full index scan
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.
fast recovery area
An optional disk location that stores recovery-related files such as control file and online redo log copies, archived redo log files, flashback logs, and RMAN backups.
foreign key
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.
full table scan
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.
grid computing
A computing architecture that coordinates large numbers of servers and storage to act as a single large computer.
grid infrastructure
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.
hard parse
The set of steps performed by Oracle 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 already exist in the shared pool.
hash function
A function that operates on an arbitrary-length input value and returns a fixed-length hash value.
hashing
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.
hash join
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.
hash table
An in-memory data structure that associates join keys with rows in a hash join. For example, in a join of the employees and 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.
heap-organized 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.
hint
An instruction passed to the optimizer through comments in a SQL statement. The optimizer uses hints to choose an execution plan for the statement.
implicit query
A component of a DML statement that retrieves data without a subquery. An UPDATE, DELETE, or MERGE statement that does not explicitly include a SELECT statement uses an implicit query to retrieve the rows to be modified.
inconsistent backup
A backup in which some files in the backup contain changes made after the checkpoint. Unlike a consistent backup, an inconsistent backup requires media recovery to be made consistent.
index
Optional schema object associated with a nonclustered table, table partition, or table cluster. In some cases indexes speed data access.
index-organized table
A table whose storage organization is a variant of a primary B-tree index. Unlike a heap-organized table, data is stored in primary key order.
initialization parameter
A configuration parameter such as DB_NAME or 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.
initialization parameter file
A text file that contains initialization parameter settings for a database instance.
instance
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.
instance failure
The termination of a database instance because of a hardware failure, Oracle internal error, or SHUTDOWN ABORT statement.
instance recovery
The automatic application of redo log records to uncommitted data blocks when an instance is restarted after a failure.
integrity constraint
Declarative method of defining a rule for a column. The integrity constraints enforce business rules and prevent the entry of invalid information into tables.
join
A statement that retrieves data from multiple tables specified in the FROM clause. Join types include inner joins, outer joins, and Cartesian joins.
join condition
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 TRUE.
key compression
The elimination of repeated occurrence of primary key column values in an index-organized table.
large pool
Optional area in the SGA that provides large memory allocations for backup and restore operations, I/O server processes, and session memory for the shared server and Oracle XA.
latch
A low-level serialization control mechanism used to protect shared data structures in the SGA from simultaneous access.
library cache
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 handles.
listener
A process that listens for incoming client connection requests and manages network traffic to the database.
locally managed tablespace
A tablespace that uses a bitmap stored in each data file to manage the extents. In contrast, a dictionary-managed tablespace uses the data dictionary to manage space.
lock
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.
log sequence number
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.
log switch
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.
log writer (LGWR)
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.
lost update
A data integrity problem in which one writer of data overwrites the changes of a different writer modifying the same data.
manual undo management mode
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.
master site
In a replication environment, a different database with which a materialized view shares data.
materialized view
A schema object that stores the result of a query. Oracle materialized views can be read-only or updatable.
See also view.
null
Absence of a value in a column of a row. Nulls indicate missing, unknown, or inapplicable data.
object type
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.
OLAP
Online Analytical Processing. OLAP is characterized by dynamic, dimensional analysis of historical data.
OLTP
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.
online redo log
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.
operator
1. In memory management, operators control the flow of data. Examples include sort, hash join, and bitmap merge operators.
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 (*).
optimizer
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.
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.
Oracle ASM disk group
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.
Oracle Clusterware
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.
Oracle Enterprise Manager
A system management tool that provides centralized management of an Oracle database environment.
Oracle process
A process that runs Oracle Database code. Oracle processes include server processes and background processes.
Oracle RAC
Oracle Real Application Clusters. Option that allows multiple concurrent database instances to share a single physical database.
Oracle XA
An external interface that allows global transactions to be coordinated by a transaction manager other than Oracle Database.
partition
A piece of a table or index that shares the same logical attributes as the other partitions. For example, all partitions in a table share the same column and constraint definitions.
PGA
Program global area. A memory buffer that contains data and control information for a server process.
See also SGA.
PL/SQL
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.
precompiler
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.
primary key
The column or set of columns that uniquely identifies a row in a table. Only one primary key can be defined for each table.
primary key constraint
An integrity constraint that disallows duplicate values and nulls in a column or set of columns.
private SQL area
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.
privilege
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.
procedure
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.
process
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.
See also background process; Oracle process; client process.
query
An operation that retrieves data from tables or views. For example, SELECT * FROM employees is a query.
See also implicit query; subquery.
read consistency
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.
See also concurrency; data consistency.
redo log
A set of files that protect altered database data in memory that has not been written to the data files. The redo log can consist of two parts: the online redo log and the archived redo log.
redo log buffer
Memory structure in the SGA that 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 file, which is used if instance recovery is necessary.
referential integrity
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).
RMAN
Recovery Manager. An Oracle Database utility that backs up, restores, and recovers Oracle databases.
row
A set of column information corresponding to a single record in a table. The database stores rows in data blocks.
row chaining
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.
row migration
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.
schema
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.
schema object
A logical structure of data stored in a schema. Examples of schema objects are tables, indexes, sequences, and database links.
SCN
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.
segment
A set of extents allocated for a specific database object such as a table, index, or table cluster. User segments, undo segments, and temporary segments are all types of segments.
selectivity
In a query, the measure of how many rows from a row set pass a predicate test, for example, WHERE last_name = 'Smith'.
server
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.
server parameter file
A server-side binary file containing initialization parameter settings that is read and written to by the database.
server process
An Oracle process that communicates with a client process and Oracle Database to fulfill user requests. The server processes are associated with a database instance, but are not part of the instance.
session
A logical entity in the database instance memory that represents the state of a current user login to a database. A single connection can have 0, 1, or more sessions established on it.
SGA
System global area. A group of shared memory structures that contain data and control information for one Oracle database instance.
shared server
A database configuration that enables multiple client processes to share a small number of server processes.
See also dedicated server.
shared SQL area
An area in the shared pool that contains the parse tree and execution plan for a SQL statement. Only one shared SQL area exists for a unique statement.
soft parse
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.
SQL
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.
standby database
An independent copy of a production database that you can use for disaster protection in a high availability environment.
statement-level rollback
A database operation in which the effects of an unsuccessful SQL statement are rolled back because the statement caused an error during execution.
stored procedure
A named PL/SQL block or Java program that Oracle Database stores in the database. Applications can call stored procedures by name.
subquery
A query nested within another SQL statement. Unlike implicit queries, subqueries use a SELECT statement to retrieve data.
synonym
An alias for a schema object. You can use synonyms to provide data independence and location transparency.
table cluster
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.
table compression
The compression of data segments to reduce disk space in a heap-organized table or table partition.
tablespace
A database storage unit that groups related logical structures together. The database data files are stored in tablespaces.
temp file
A file that belongs to a temporary tablespace. The temp files in temporary tablespaces cannot contain permanent database objects.
temporary segment
A segment created by Oracle Database when a SQL statement needs a temporary database area to complete execution.
temporary table
A table that holds an intermediate result set for the duration of a transaction or a session. Only the current session can see the data in temporary tables.
transaction
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.
transaction ID
An identifier is unique to a transaction and represents the undo segment number, slot, and sequence number.
transaction recovery
A phase of instance recovery in which uncommitted transactions are rolled back.
transaction table
The data structure within an undo segment that holds the transaction identifiers of the transactions using the undo segment.
transportable tablespace
A tablespace that you can copy or move between databases. Oracle Data Pump provides the infrastructure for transportable tablespaces.
trigger
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.
UGA
User global area. Session memory that stores session variables, such as logon information, and can also contain the OLAP pool.
undo data
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.
undo tablespace
A tablespace containing undo segments when automatic undo management mode is enabled.
user name
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.
view
A custom-tailored presentation of the data in one or more tables. The views do not actually contain or store data, but derive it from the tables on which they are based.
virtual column
A column that is not stored on disk. The database derives the values in virtual columns on demand by computing a set of expressions or functions.
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