The data dictionary of an ORACLE database is a set of tables and views that are used as a read-only reference about the database.
It stores information about both the logical and physical structure of the database, the valid users of an ORACLE database, integrity constraints defined for tables in the database and space allocated for a schema object and how much of it is being used.
The Oracle data dictionary is one of the most important components of the Oracle DBMS. It contains all information about the structures and objects of the database such as tables columns users data files etc. The data stored in the data dictionary are also often called metadata. Although it is usually the domain of database administrators (DBAs) the data dictionary is a valuable source of information for end users and developers. The data dictionary consists of two levels: the internal level contains all base tables that are used by the various DBMS software components and they are normally not accessible by end users. The external level provides numerous views on these base tables to access information about objects and structures at different levels of detail.
Oracle continously requests and update the information in the data dictionary of the database.To minimize the performance of the system internal operation data dictionary cache holds the dictionary information.
Data Dictionary is a read-only set of tables that provides information about the database. in order to avoid the I/O cost these data dictionary views are used
