Throughout its operation, Oracle maintains a set of virtual tables that record current
database activity. These tables are called dynamic performance tables.

Dynamic performance tables are not true tables, and they should not be accessed by
most users. However, database administrators can query and create views on the
tables and grant access to those views to other users. These views are sometimes called
fixed views because they cannot be altered or removed by the database administrator.

SYS owns the dynamic performance tables; their names all begin with V_$. Views are
created on these tables, and then public synonyms are created for the views. The
synonym names begin with V$. For example, the V$DATAFILE view contains
information about the database’s datafiles, and the V$FIXED_TABLE view contains
information about all of the dynamic performance tables and views in the database.

动态性能表
1. 当数据库运行时 ，Oracle将维护一组虚拟表用于记录数据库的当前状态 , 称为动态性能表
2. 动态性能视图并不是真正的表
3. 动态性能表由 SYS 用户所拥有；她们的名字都以 V_$开头。这些表上创建有视图，而视图上创
建有公共同义词。这些公共同义词的名字都以 V$ 开头