Oracle s Data Dictionary


Oracle's Data Dictionary

During the creation of the database, the Oracle Server creates additional physical objects in the datafiles and logical structures within the instance. These structures assist in the running of the database and the location of other structures.

These structures include a set of read-only tables and views that record and provide verification and other information about the database to which it belongs.

In the data dictionary there are base tables that store the description of the database and its components. These base tables are created behind the scenes automatically when you issue the CREATE DATABASE command (Oracle runs sql.bsq to create them). They are the first structures in any Oracle database. The Oracle Server writes to these tables whenever there is a need to update any of them. It is the only thing that should ever write to these tables; no data definition language (DDL) or data manipulation language (DML) should ever be run to update any of these tables with the exception, possibly, of the AUD$ tables that are connected to auditing.

More information on the AUD$ tables can be found in Chapter 17, "Managing Privileges."


Also inside the data dictionary are the data dictionary views that summarize and compartmentalize the base table information. These views allow for a more useful and formatted presentation, including object names rather than simply the stored object numbers. These read-only views are created when you run the catalog.sql script located in the $ORACLE_HOME/rdbms/admin (for UNIX) or %ORACLE_HOME%\rdbms\admin (on Windows) directory.

Through these tables and views, you can learn more about the definitions and names, definitions and space allocations of objects, and integrity constraints on the various objects. You can learn about users, their roles and privileges, and information about auditing these users' actions.

The data dictionary contains the following:

  • The definitions of all the schemas in the database

  • The definitions of the tables, views, indexes, clusters, synonyms, sequences, procedures, packages, functions, triggers, and other schema objects

  • The space that has been allocated and used by each of the schema objects in the database

  • The default values for columns in the tables in the database

  • The integrity constraints for objects in the database

  • The check and not null constraints for objects in the database

  • The userid (name) of Oracle users

  • The privileges and roles that have been granted to each Oracle user

  • The auditing information that might exist that is connected to anyone who has accessed, or updated, a variety of defined schema objects

The data dictionary keeps track of every aspect of your database. It keeps track of tables and users, tablespaces and objects, space allocation, and every connection that has been made to the database. It is a key component of the database and should be treated as such.


The data dictionary is located in the system tablespace and usually needs to be re-created when you upgrade the database from one version of the server to another.



    Oracle 9i Fundamentals I Exam Cram 2
    Oracle 9i Fundamentals I Exam Cram 2
    ISBN: 0789732653
    EAN: 2147483647
    Year: 2004
    Pages: 244
    Authors: April Wells

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