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All the objects / views starts with V$ or GV$ are called dynamic performance views. As the name indicates these views are getting updated frequently or it is up-to-date with database statistics and performance statistics. Oracle internally manages this views and no other user has manipulation access on these views. Other users can get read only privilege on these views.
V$ views are the public synonyms on the V_$ objects. V_$ objects are the actual dynamic performance views. By creating a V$ public synonyms oracle maintains an additional layer security.
If you check in Oracle RAC databases, apart from V$ views you can see another set of tables which starts with GV$. These views are for Oracle RAC dynamic performance views. These views contain an additional column (inst_id) to identify each instance. For example, If you have 4 node RAC database and all 4 nodes are running, in each GV$ view you can 4 different rows with 1 to 4 instance IDs.
SQL> select inst_id, instance_name from gv$instance;
INST_ID INSTANCE_NAME
---------- ----------------
2 PRODF2
3 PRODF3
In the above RAC database instance 1 is down and it is showing the 2nd and 3rd instances.
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