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 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.
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.
A database's data dictionary consists of:
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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. |
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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. |
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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.
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WARNING: Altering or manipulating the data in underlying data dictionary tables can permanently and detrimentally affect the operation of a database. |
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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.
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 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.
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:
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Prefix |
Scope |
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USER |
user's view (what is in the user's schema) |
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ALL |
expanded user's view (what the user can access) |
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DBA |
database administrator's view (what is in all users' schemas) |
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The set of columns is identical across views with these exceptions:
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The views most likely to be of interest to typical database users are those with the prefix USER. These views
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 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 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 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".