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The Data Dictionary

LEXICOGRAPHER--A writer of dictionaries, a harmless drudge.

Samuel Johnson: Dictionary

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:

An Introduction to the Data Dictionary

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 its associated database. A data dictionary contains:

the definitions of all schema objects in the database (tables, views, indexes, clusters, synonyms, sequences, procedures, functions, packages, triggers, and so on)
how much space has been allocated for, and is currently used by, the schema objects
default values for columns
integrity constraint information
the names of Oracle users
privileges and roles each user has been granted
auditing information, such as who has accessed or updated various schema objects
other general database information

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 SYSTEM tablespace (see "The SYSTEM Tablespace").

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, you 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 Structure of the Data Dictionary

A database's data dictionary consists of:

base tables

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.

user-accessible views

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 WHERE clauses to simplify the information. Most users are given access to the views rather than the base tables.

SYS, the Owner of the Data Dictionary

The Oracle user SYS owns all base tables and user-accessible views of the data dictionary. Therefore, 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 should keep strict control of this central account.

WARNING:
Altering or manipulating the data in underlying data dictionary tables can permanently and detrimentally affect the operation of a database.

How the Data Dictionary Is Used

The data dictionary has three primary uses:

Oracle accesses the data dictionary to find information about users, schema objects, and storage structures.
Oracle modifies the data dictionary every time that a data definition language (DDL) statement is issued.
Any Oracle user can use the data dictionary as a read-only reference for information about the database.

How Oracle Uses the Data Dictionary

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.

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.

Public Synonyms for Data Dictionary Views

Oracle creates public synonyms on many data dictionary views so that 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.

Caching of the Data Dictionary for Fast Access

Much of the data dictionary information is cached in the SGA (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 Programs and the Data Dictionary

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.

Adding New Data Dictionary Items

You can add new tables or views to the data dictionary. If you add new data dictionary objects, the owner of the new objects should be the user SYSTEM or a third Oracle user.

Caution:
Never create new objects belonging to user SYS, except by running the script provided by Oracle Corporation for creating data dictionary objects.

Deleting Data Dictionary Items

All changes to the data dictionary are performed by Oracle in response to DDL statements, therefore no data in any data dictionary tables should be deleted or altered by any user.

The single exception to this rule is the table SYS.AUD$. When auditing is enabled, this table can grow without bound. Although you should not drop the AUDIT_TRAIL table, the security administrator can safely delete data from it because the rows are for information only and are not necessary for Oracle to run.

How Users and DBAs Can Use the Data Dictionary

The views of the data dictionary serve as a reference for all database users. You access the data dictionary views via the SQL language. Some views are accessible to all Oracle users; 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:

Table 2-1 Data Dictionary View Prefixes

Prefix	Scope
USER	user's view (what is in the user's schema)
ALL	expanded user's view (what the user can access)
DBA	database administrator's view (what is in all users' schemas)

The set of columns is identical across views with these exceptions:

Views with the prefix USER usually exclude the column OWNER. This column is implied in the USER views to be the user issuing the query.
Some DBA views have additional columns containing information useful to the administrator.

Additional Information:
See the Oracle8i Reference for a complete list of data dictionary views and their columns.

Views with the Prefix USER

The views most likely to be of interest to typical database users are those with the prefix USER. These views

refer to the user's own private environment in the database, including information about schema objects created by the user, grants made by the user, and so on
display only rows pertinent to the user
have columns identical to the other views, except that the column OWNER is implied (the current user)
return a subset of the information in the ALL_ views
can have abbreviated PUBLIC synonyms for convenience

For example, the following query returns all the objects contained in your schema:

SELECT object_name, object_type FROM user_objects;

Views with the Prefix ALL

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 via 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;

Views with the Prefix DBA

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, SYS, as in

SELECT owner, object_name, object_type FROM sys.dba_objects;

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 DUAL Table

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 "X".

Additional Information:
See the description of the SELECT command in the Oracle8i SQL Reference for more information about the DUAL table.

The Dynamic Performance Tables

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, V$DATAFILE contains information about the database's datafiles and V$FIXED_TABLE contains information about all of the dynamic performance tables and views in the database.

Additional Information:
See the Oracle8i Reference for a complete list of the dynamic performance views' synonyms and their columns.

base tables	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.
user-accessible views	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 WHERE clauses to simplify the information. Most users are given access to the views rather than the base tables.

2 The Data Dictionary