data dictionary | Oracle SQL | data dictionary in sql | Oracle dictionary
In this session , will learn about data dictionary in Oracle.
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What are dictionary tables in Oracle?
A data dictionary in Oracle is a collection of read-only tables that provide useful information about the database including schemas, users, privileges, and even auditing data. The values in these stored dictionaries are updated automatically by Oracle anytime a statement is executed on the server that modifies data.
Introduction to the Data Dictionary
One of the most important parts of an Oracle database is its 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.
Structure of the Data Dictionary
The data dictionary consists of the following:
Base Tables
The underlying tables that store 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.
Prefix
USER_ Only shows objects that are owned by the user that selects from that view
ALL_ Shows objects on which the querying user has been granted privileges
DBA_ All objects, sometimes with additional columns that are interesting for DBAs.
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