Dynamic performance views are sometimes called fixed views because they cannot be altered or removed by a database administrator. 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 almost every V$ view, a corresponding GV$ view exists. In Oracle Real Application Clusters (Oracle RAC), querying a GV$ view retrieves the V$ view information from all qualified database instances 上面是官方concepts的解释。没有提到X$表。X$表就是内存中的虚拟表，即所谓的dynamic performance tables，在它上面建立的动态性能视图名称以V_$开头，每个V_$开头的视图有对应的V$公共同义词。几乎每个V$视图都有一个相应的GV$视图，主要用在RAC中，可以从中查到所有实例中的V$信息
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 almost every V$ view, a corresponding GV$ view exists. In Oracle Real Application Clusters (Oracle RAC), querying a GV$ view retrieves the V$ view information from all qualified database instances 上面是官方concepts的解释。没有提到X$表。X$表就是内存中的虚拟表，即所谓的dynamic performance tables，在它上面建立的动态性能视图名称以V_$开头，每个V_$开头的视图有对应的V$公共同义词。几乎每个V$视图都有一个相应的GV$视图，主要用在RAC中，可以从中查到所有实例中的V$信息