What is Data Dictionary

Data dictionary 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 importanttool 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.

Structure of the Data Dictionary
The data dictionary consists of the following:

Base Tables stores 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, Owner of the Data Dictionary The Oracle user SYS owns all base tables and user-accessible views of the data dictionary. 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 must keep strict control of this central account.

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.