|Oracle9i Database Concepts
Release 1 (9.0.1)
Part Number A88856-02
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 includes:
One of the most important parts of an Oracle database is its data dictionary, which is a read-only set of tables that provides information about the database. A data dictionary contains:
The data dictionary is structured in tables and views, just like other database data. All the data dictionary tables and views for a given database are stored in that database's
Not only is the data dictionary central to every Oracle database, it is an important tool for all users, from end users to application designers and database administrators. To access the data dictionary, use SQL statements. Because the data dictionary is read-only, you can issue only queries
(SELECT statements) against the tables and views of the data dictionary.
"The SYSTEM Tablespace" for more information about
A database's data dictionary consists of:
The underlying tables that store information about the associated database. Only Oracle should write to and read these tables. Users rarely access them directly because they are normalized, and most of the data is stored in a cryptic format.
The views that summarize and display the information stored in the base tables of the data dictionary. These views decode the base table data into useful information, such as user or table names, using joins and
The Oracle user
SYS owns all base tables and user-accessible views of the data dictionary. No Oracle user should ever alter (update, delete, or insert) any 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.
The data dictionary has three primary uses:
Data in the base tables of the data dictionary is necessary for Oracle to function. Therefore, only Oracle should write or change data dictionary information. Oracle provides scripts to modify the data dictionary tables when a database is upgraded or downgraded.
During database operation, Oracle reads the data dictionary to ascertain that schema objects exist and that users have proper access to them. Oracle also updates the data dictionary continuously to reflect changes in database structures, auditing, grants, and data.
For example, if user
KATHY creates a table named
PARTS, new rows are added to the data dictionary that reflect the new table, columns, segment, extents, and the privileges that
KATHY has on the table. This new information is then visible the next time the dictionary views are queried.
Oracle creates public synonyms on 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.
Much of the data dictionary information is cached in the SGA in the dictionary cache, because Oracle constantly accesses the data dictionary during database operation to validate user access and to verify the state of schema objects. All information is stored in memory using the LRU (least recently used) algorithm.
Information typically kept in the caches is that required for parsing. The
COMMENTS columns describing the tables and their columns are not cached unless they are accessed frequently.
Other Oracle 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: the synonyms are less likely to change between software releases.
The views of the data dictionary serve as a reference for all database users. You access the data dictionary views through the SQL language. Some views are accessible to all Oracle users, and others are intended for database administrators only.
The data dictionary is always available when the database is open. It resides in the
SYSTEM tablespace, which is always online.
The data dictionary consists of sets of views. In many cases, a set consists of three views containing similar information and distinguished from each other by their prefixes:
user's view (what is in the user's schema)
expanded user's view (what the user can access)
database administrator's view (what is in all users' schemas)
The set of columns is identical across views with these exceptions:
USERusually exclude the column
OWNER.This column is implied in the
USERviews to be the user issuing the query.
DBAviews have additional columns containing information useful to the administrator.
The views most likely to be of interest to typical database users are those with the prefix
USER. These views:
OWNERis implied (the current user)
PUBLICsynonyms for convenience
For example, the following query returns all the objects contained in your schema:
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:
Views with the prefix
DBA show a global view of the entire database. Therefore, they are meant to be queried only by database administrators. Any user granted the system privilege
SELECT ANY TABLE can query the
DBA-prefixed views of the data dictionary.
Synonyms are not created for these views, because the
DBA views should be queried only by administrators. Therefore, to query the
DBA views, administrators must prefix the view name with its owner,
Administrators can run the script file
DBA_SYNONYMS.SQL to create private synonyms for the
DBA views in their accounts if they have the
SELECT ANY TABLE system privilege. Executing this script creates synonyms for the current user only.
The table named
DUAL is a small table in the data dictionary that Oracle and user-written programs can reference to guarantee a known result. This table has one column called
DUMMY and one row containing the value
Oracle9i SQL Reference for more information about the
Throughout its operation, Oracle maintains a set of "virtual" tables that record current database activity. These tables are called dynamic performance tables.
Dynamic performance tables are not true tables, and they should not be accessed by most users. However, database administrators can query and create views on the tables and grant access to those views to other users. These views are sometimes called fixed views because they cannot be altered or removed by the database administrator.
SYS owns the dynamic performance tables; their names all begin with
V_$. Views are created on these tables, and then public synonyms are created for the views. The synonym names begin with
V$. For example, the
V$DATAFILE view contains information about the database's datafiles, and the
V$FIXED_TABLE view contains information about all of the dynamic performance tables and views in the database.
Oracle9i Database Reference for a complete list of the dynamic performance views' synonyms and their columns
Oracle9i, Release 1 (9.0.1), includes a PL/SQL package,
DBMS_METADATA, which 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: