Oracle Database Uses the Data Dictionary
The Oracle Database user SYS owns all base tables and user-accessible views of the data dictionary. Data in the base tables of the data dictionary is necessary for Oracle Database to function. Therefore, only Oracle Database should write or change data dictionary information. No Oracle Database user should ever alter 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.
Caution: Altering or manipulating the data in data dictionary tables can permanently and detrimentally affect database operation.
Overview of the Dynamic Performance Views
During database operation, Oracle Database reads the data dictionary to ascertain that schema objects exist and that users have proper access to them. Oracle Database also updates the data dictionary continuously to reflect changes in database structures, auditing, grants, and data.
Public Synonyms for Data Dictionary Views
Oracle Database creates public synonyms for many data dictionary views so users can access them conveniently. The security administrator can also create additional public synonyms for schema objects that are used system wide. Users should avoid naming their own schema objects with the same names as those used for public synonyms.
Cache the Data Dictionary for Fast Access
Much of the data dictionary information is in the data dictionary cache because the database constantly requires the information to validate user access and verify the state of schema objects. Parsing information is typically kept in the caches. The COMMENTS columns describing the tables and their columns are not cached in the dictionary cache, but may be cached in the database buffer cache.
Throughout its operation, Oracle Database maintains a set of virtual tables that record current database activity. These views are called dynamic performance views because they are continuously updated while a database is open and in use. The views, also sometimes called V$ views, contain information such as the following:
■System and session parameters
■Memory usage and allocation
■File states (including RMAN backup files)
■Progress of jobs and tasks
■Statistics and metrics
Contents of the Dynamic Performance Views
Dynamic performance views are sometimes called fixed views because they cannot be altered or removed by a database administrator. However, database administrators can query and create views on the tables and grant access to these views to other users.SYS owns the dynamic performance tables, whose names begin with V_$. Views are created on these tables, and then public synonyms prefixed with V$. For example, the V$ DATA FILE view contains information about data files.
When you use the Database Configuration Assistant (DBCA) to create a database, Oracle automatically creates the data dictionary. Oracle Database automatically runs the catalog. Sql script,which contains definitions of the views and public synonyms for the dynamic performance views.