The data dictionary consists of base tables and views.

These objects are defined as follows:

• Base tables

  These 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.

• Views

  These decode the base table data into useful information, such as user or table names, using joins and WHERE clauses to simplify the information. The 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 the following table. By querying the appropriate views, you can access only the information relevant for you.

Not all views sets have three members. For example, the data dictionary contains a DBA_LOCK view but no ALL_LOCK view.

The system-supplied 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
.
.
.

SELECT OWNER, OBJECT_NAME, OBJECT_TYPE
FROM   DBA_OBJECTS
ORDER BY OWNER, OBJECT_NAME;

SELECT OWNER, OBJECT_NAME, OBJECT_TYPE 
FROM   ALL_OBJECTS
ORDER BY OWNER, OBJECT_NAME; 

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

SELECT OBJECT_NAME, OBJECT_TYPE 
FROM   USER_OBJECTS
ORDER BY OBJECT_NAME;

SQL> SELECT ((3*4)+5)/3 FROM DUAL;
 
((3*4)+5)/3
-----------
 5.66666667

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.

The Oracle Database user account SYS owns all base tables and user-accessible views of the data dictionary.

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 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 the database adds new rows 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.

Data in the base tables of the data dictionary is necessary for Oracle Database to function. 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.

Dynamic performance views are 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.

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.

Dynamic performance views are based on virtual tables built from database memory structures.

The views 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$INSTANCE and V$BGPROCESS when the database is started but not mounted. However, you cannot query V$DATAFILE until the database has been mounted.