Data Dictionary Tables in RDBMS

In a Relational Database Management System (RDBMS), a data dictionary is a collection of READ-ONLY tables and views that contain metadata about the database. It typically contains data about all users in the system, their assigned roles and privileges, database objects, their structure, relationship, integrity constraints, default values and storage space details, auditing information, and many more. Since, it is READ-ONLY mode, only SELECT operation can be done on tables in the Data Dictionary.

Oracle Data Dictionary Structure

Oracle DBMS's data dictionary is comprised of 2 types of objects:

1. Base Tables: Base Tables are the foundation of the Data Dictionary. It contains information about the database to which it is associated. It is READ-only, and only the Oracle server or processing engine can access it Users can't access it. The data in that table are stored in normalized form and cryptic formats.
2. User Accessible Views: formatsThe data dictionary also contains views that are accessible to the user. It contains the information about base tables in a convenient way. They are extracted decoded versions of the Base table which contains useful information like user or table names. Generally, all users have access to this, unlike Base tables.

Both these tables and views are owned by SYS. No user should edit it as it can adversely affect the database.

Uses of Data Dictionary

The Oracle engine accesses this data dictionary whenever a DDL statement is evaluated. It is essential for any database to function. Its uses include Database administration, Query Optimization, Metadata management, Security and access control, and much more.

User Accessible Views

Let's see how DBMS users can use these Data Dictionary views.

1. User-accessible views can be viewed by all users using SQL query depending on the permission given by the Database Admin.
2. Data Dictionary views are classified into three types by their prefixes. Each Prefix and its scope are presented in the table below.

USER Views

The views with the prefix USER generally refer to views that contain data about a particular user's own object, which includes data about objects created by that user, grants made by that user, and so on.

It is generally a subset of ALL_views. It has only data related to the user and has columns similar to other views.

Examples:

Some Common Views with the prefix USER are:

USER_OBJECTS - view that contains data about all objects
(tables, views, indexes, procedures, and functions, etc.) owned by that particular user
USER_TAB_COLUMNS - view that contains data about the columns of all 
tables and views owned by that particular user.
USER_TABLES - view that contains data about all tables owned by that particular user.
USER_VIEWS - view that contains data about all views owned by that particular user.
USER_CONSTRAINTS - view that contains data about all constraints 
(primary keys, foreign keys, and unique constraints, etc.) owned by the particular user.
USER_SEQUENCES - view that contains data about all 
sequences owned by that particular user.
USER_TRIGGERS - view that contains data about all triggers 
owned by that particular user.
USER_INDEXES - view that contains data about all indexes owned by that particular user.
USER_SYNONYMS - view that contains data about all synonyms owned by that particular user.
USER_TAB_COMMENTS - view that contains comments on tables and views owned by that particular user.

Example Query To Access the Views

Query to print all the objects (name and type) in a user's scheme

Syntax:

SELECT object_name, object_type FROM user_objects; 

Query to print all the data about all Tables owned by the user:

SELECT * FROM user_tables;

ALL Views

Views with the prefix ALL are views that contain data about not only objects owned by the user but also objects with access via public or explicit grants of privileges or roles.

Examples:

Some Common Views with the prefix ALL are:

ALL_OBJECTS - views that contains data about all objects (tables, views, indexes, procedures, etc..) in the database
ALL_TAB_COLUMNS - views that contains data about all columns in all the tables and views in the database
ALL_TABLES - views that contains data about all tables and views in the database
ALL_VIEWS - views that contains data about all views in the database in the database
ALL_USERS - Views that contains data about data about all users in the database
ALL_CONSTRAINTS - Views that contains data about all constraints 
(primary keys, foreign keys, unique constraints, etc. ) in the database
ALL_SEQUENCES - View that contains data about all sequences in the database
ALL_INDEXES - Views that contains data about all indexes in the database
ALL_TRIGGERS - Views that contains data about about all triggers in the database
ALL_INDEXES: View that contains data about all indexes in the database.
ALL_SYNONYMS - Views that contains data about all synonyms in the database

Example:

Query to print all the objects (name and type) to which the user has access:

SELECT owner, object_name, object_type FROM all_objects; 

Query to print all the data about all Tables to which the user has access:

SELECT * FROM all_views;

DBA Views

Views with the prefix DBA are views that are generally accessed only by the Database Administrators or any user who has got the system privilege SELECT ANY TABLE. It will have access to the OWNER column as well. The prefix is generally SYS followed by the table name.

The views are similar to the ALL and USER views

Example:

Query to print all the objects (name and type) in the DataBase:

SELECT owner, object_name, object_type FROM sys.dba_objects;