The dynamic performance views (the V$ views) mainly provide information about the Oracle instance, as well as information that the instance maintains about the database. The views in this category are considered dynamic, because in general, their contents change based upon how the instance is performing. The contents of these views are representative of the total instance or cluster workload, rather than any one specific SQL statement.
Specific dynamic performance views are available based on the status of the instance, as follows:
Those views that provide information specifically about the instance (e.g., V$PARAMETER) are available immediately once the instance is started.
Those views that provide information stored in the control files are available once the database has been mounted (as noted in the view description).
Those views that provide information about how the kernel is processing SQL statements are available once the database has been opened (as noted in the view description).
Unlike the static data dictionary views, which are views on existing tables, the dynamic performance data dictionary views are views on a set of tables that do not physically exist in the database; instead they are actually views on X$ tables, which in turn are representations of internal memory structures in the Oracle instance. Views are constructed as follows: