|Oracle® Database Concepts
12c Release 1 (12.1)
|PDF · Mobi · ePub|
This chapter describes the central set of read-only reference tables and views of each Oracle database, known collectively as the data dictionary. The chapter also describes the dynamic performance views, which are special views that are continuously updated while a database is open and in use.
This chapter contains the following sections:
An important part of an Oracle database is its data dictionary, which is a read-only set of tables that provides administrative metadata about the database. A data dictionary contains information such as the following:
The definitions of every schema object in the database, including default values for columns and integrity constraint information
The amount of space allocated for and currently used by the schema objects
The names of Oracle Database users, privileges and roles granted to users, and auditing information related to users (see "User Accounts")
The data dictionary is a central part of data management for every Oracle database. For example, the database performs the following actions:
Accesses the data dictionary to find information about users, schema objects, and storage structures
Modifies the data dictionary every time that a DDL statement is issued (see "Data Definition Language (DDL) Statements")
Because Oracle Database stores data dictionary data in tables, just like other data, users can query the data with SQL. For example, users can run
SELECT statements to determine their privileges, which tables exist in their schema, which columns are in these tables, whether indexes are built on these columns, and so on.
See Also:"Introduction to Schema Objects"
These underlying tables store information about the database. Only Oracle Database should write to and read these tables. Users rarely access the base tables directly because they are normalized and most data is stored in a cryptic format.
These views decode the base table data into useful information, such as user or table names, using joins and
WHERE clauses to simplify the information. These views contain the names and description of all objects in the data dictionary. Some views are accessible to all database users, whereas others are intended for administrators only.
Typically, data dictionary views are grouped in sets. In many cases, a set consists of three views containing similar information and distinguished from each other by their prefixes, as shown in Table 6-1. By querying the appropriate views, you can access only the information relevant for you.
Objects to which user has privileges
Includes objects owned by user. These views obey the current set of enabled roles.
Objects owned by user
Views with the prefix
Not all views sets have three members. For example, the data dictionary contains a
DBA_LOCK view but no
DICTIONARY view contains the names and abbreviated descriptions of all data dictionary views. The following query of this view includes partial sample output:
SQL> SELECT * FROM DICTIONARY 2 ORDER BY TABLE_NAME; TABLE_NAME COMMENTS ------------------------------ ---------------------------------------- ALL_ALL_TABLES Description of all object and relational tables accessible to the user ALL_APPLY Details about each apply process that dequeues from the queue visible to the current user . . .
For example, the following query shows information about all objects in the database:
SELECT OWNER, OBJECT_NAME, OBJECT_TYPE FROM DBA_OBJECTS ORDER BY OWNER, OBJECT_NAME;
See Also:Oracle Database Administrator's Guide for detailed information on administrative privileges
Views with the prefix
ALL_ refer to the user's overall perspective of the database. These views return information about schema objects to which the user has access through public or explicit grants of privileges and roles, in addition to schema objects that the user owns.
For example, the following query returns information about all the objects to which you have access:
SELECT OWNER, OBJECT_NAME, OBJECT_TYPE FROM ALL_OBJECTS ORDER BY OWNER, OBJECT_NAME;
ALL_ views obey the current set of enabled roles, query results depend on which roles are enabled, as shown in the following example:
SQL> SET ROLE ALL; Role set. SQL> SELECT COUNT(*) FROM ALL_OBJECTS; COUNT(*) ---------- 68295 SQL> SET ROLE NONE; Role set. SQL> SELECT COUNT(*) FROM ALL_OBJECTS; COUNT(*) ---------- 53771
Application developers should be cognizant of the effect of roles when using
ALL_ views in a stored procedure, where roles are not enabled by default.
See Also:"PL/SQL Subprograms"
Refer to the user's private environment in the database, including metadata about schema objects created by the user, grants made by the user, and so on
Display only rows pertinent to the user, returning a subset of the information in the
Has columns identical to the other views, except that the column
OWNER is implied
For example, the following query returns all the objects contained in your schema:
SELECT OBJECT_NAME, OBJECT_TYPE FROM USER_OBJECTS ORDER BY OBJECT_NAME;
DUAL is a small table in the data dictionary that Oracle Database and user-written programs can reference to guarantee a known result. The dual table is useful when a value must be returned only once, for example, the current date and time. All database users have access to
DUAL table has one column called
DUMMY and one row containing the value
X. The following example queries
DUAL to perform an arithmetical operation:
SQL> SELECT ((3*4)+5)/3 FROM DUAL; ((3*4)+5)/3 ----------- 5.66666667
See Also:Oracle Database SQL Language Reference for more information about the
The data dictionary base tables are the first objects created in any Oracle database. All data dictionary tables and views for a database are stored in the
SYSTEM tablespace. Because the
SYSTEM tablespace is always online when the database is open, the data dictionary is always available when the database is open.
See Also:"The SYSTEM Tablespace" for more information about the
The Oracle Database user
SYS owns all base tables and user-accessible views of the data dictionary. Data in the base tables of the data dictionary is necessary for Oracle Database to function. Therefore, only Oracle Database should write or change data dictionary information. No Oracle Database user should ever alter rows or schema objects contained in the
SYS schema because such activity can compromise data integrity. The security administrator must keep strict control of this central account.
Altering or manipulating the data in data dictionary tables can permanently and detrimentally affect database operation.
During database operation, Oracle Database reads the data dictionary to ascertain that schema objects exist and that users have proper access to them. Oracle Database also updates the data dictionary continuously to reflect changes in database structures, auditing, grants, and data.
For example, if user
hr creates a table named
interns, then new rows are added to the data dictionary that reflect the new table, columns, segment, extents, and the privileges that
hr has on the table. This new information is visible the next time the dictionary views are queried.
See Also:"SYS and SYSTEM Schemas"
Oracle Database creates public synonyms for many data dictionary views so users can access them conveniently. The security administrator can also create additional public synonyms for schema objects that are used systemwide. Users should avoid naming their own schema objects with the same names as those used for public synonyms.
See Also:"Overview of Synonyms"
Much of the data dictionary information is in the data dictionary cache because the database constantly requires the information to validate user access and verify the state of schema objects. Parsing information is typically kept in the caches. The
COMMENTS columns describing the tables and their columns are not cached in the dictionary cache, but may be cached in the database buffer cache.
See Also:"Data Dictionary Cache"
Other Oracle Database products can reference existing views and create additional data dictionary tables or views of their own. Application developers who write programs that refer to the data dictionary should refer to the public synonyms rather than the underlying tables. Synonyms are less likely to change between releases.
Throughout its operation, Oracle Database maintains a set of virtual tables that record current database activity. These views are dynamic because they are continuously updated while a database is open and in use. The views, also sometimes called V$ views because their names begin with
V$, contain information such as the following:
System and session parameters
Memory usage and allocation
File states (including RMAN backup files)
Progress of jobs and tasks
Administrators can use the views for performance monitoring and debugging.
See Also:Oracle Database Reference for a complete list of the dynamic performance views
Dynamic performance views are sometimes called fixed views because they cannot be altered or removed by a database administrator. However, database administrators can query and create views on the tables and grant access to these views to other users.
SYS owns the dynamic performance tables, whose names begin with
V_$. Views are created on these tables, and then public synonyms prefixed with
V$. For example, the
V$DATAFILE view contains information about data files. The
V$FIXED_TABLE view contains information about all of the dynamic performance tables and views.
For almost every
V$ view, a corresponding
GV$ view exists. In Oracle Real Application Clusters (Oracle RAC), querying a
GV$ view retrieves the
V$ view information from all qualified database instances (see "Database Server Grid").
When you use the Database Configuration Assistant (DBCA) to create a database, Oracle automatically creates the data dictionary. Oracle Database automatically runs the
catalog.sql script, which contains definitions of the views and public synonyms for the dynamic performance views. You must run
catalog.sql to create these views and synonyms.
"Tools for Database Installation and Configuration" to learn about DBCA
Oracle Database Administrator's Guide to learn how to run
Oracle Real Application Clusters Administration and Deployment Guide to learn about using performance views in Oracle RAC
Dynamic performance views are based on virtual tables built from database memory structures. Thus, they are not conventional tables stored in the database. Read consistency is not guaranteed for the views because the data is updated dynamically.
Because the dynamic performance views are not true tables, the data depends on the state of the database and database instance. For example, you can query
V$BGPROCESS when the database is started but not mounted. However, you cannot query
V$DATAFILE until the database has been mounted.
DBMS_METADATA package provides interfaces for extracting complete definitions of database objects. The definitions can be expressed either as XML or as SQL DDL. Two styles of interface are provided: a flexible, sophisticated interface for programmatic control, and a simplified interface for ad hoc querying.
See Also:Oracle Database PL/SQL Packages and Types Reference for more information about