The Oracle data dictionary is a collection of tables and related views that enable you to see the inner workings and structure of the Oracle database. By querying these tables and views, you can obtain information about every object and every user of the database. For example, you can determine the amount of I/O to each datafile, the values of the INIT.ORA parameters, and much more. All of the Oracle monitoring tools look at the information available in the data dictionary and present it in an easy-to-use format.

Traditionally, the data dictionary has consisted of a series of views owned by SYS. These views, known as static data dictionary views , present information contained in tables that are updated when Oracle processes a DDL statement. The SYS tables and views, as well as a set of public synonyms for the views, are all created by the catalog.sql script. In addition, the installation of some Oracle features creates tables and views in the SYSTEM schema. In general, tables and views owned by SYSTEM exist to support functionality provided by PL/SQL stored procedures rather than fundamental Oracle functionality.

An additional set of views is composed of the dynamic performance data dictionary views , commonly referred to as the V$ views (or, mistakenly, as the V$ tables). These V$ views are based on a set of internal memory structures maintained by Oracle as virtual tables, which all begin with an “X$” prefix. Just as the static data dictionary views provide information about the database, the V$ views and the underlying X$ tables provide information about the instance.

Table 14.1 distinguishes between the types of information you will find in the static data dictionary views and in the dynamic performance data dictionary views.