What is Data Dictionary?
Database: Any collection of information; technically, a collection of computer data in related records treated as a single unit. Data Dictionary is the information about database.
The data dictionary is where Oracle stores information about the structure of the database. The data itself is located in other ares- the data dictionary describe how the actual data is organized. The dictionary consits of tables and views that you can query, like any other database table or view. The views are owned by the Oracle user SYS.
The data dictionary is typically created when the database is created and installed for the first time. Without the data dictionary, no PL/SQL work can be done. On most systems, there is a script called catproc.sql that creates the dictionary views. This script should be run while connected as SYS, or connected as internal in SQL DBA or Server Manager.
In addition to creating the data dictionary itself, catproc.sql creates the standard PL/SQL and DBMS packages, which are stored in the data dictionary.
Data dictionary, which is a read-only set of tables that provides information about the 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.
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. Use SQL statements to access the data dictionary. Because the data dictionary is read only, you can issue only queries (SELECT statements) against it's tables and views.