Oracle contains a set of underlying views that are maintained by the database server and accessible to the database administrator user SYS. These views are called dynamic performance views because they are continuously updated while a database is open and in use, and their contents relate primarily to performance.
Although these views appear to be regular database tables, they are not. These views provide data on internal disk structures and memory structures. You can select from these views, but you can never update or alter them.
Note:
You can query the dynamic performance views to extract information from them. However, only simple queries are supported. If sorts, joins, GROUP BY clauses and the like are needed, then you should copy the information from each V$ view into a table (for example, using a CREATE TABLE ... AS SELECT statement), and then query from those tables.
Because the information in the V$ views is dynamic, read consistency is not guaranteed for SELECT operations on these views.
The catalog.sql script contains definitions of the views and public synonyms for the dynamic performance views. You must run catalog.sql to create these views and synonyms. After installation, only user SYS or anyone with SYSDBA privilege has access to the dynamic performance tables. See Oracle Database Administrator's Guide for more information about running catalog.sql.