Oracle Performance Views - V$ and GV$

V$ Views

The actual dynamic performance views are identified by the prefix V_$. Public synonyms for these views have the prefix V$. Database administrators and other users should access only the V$ objects, not the V_$ objects. The dynamic performance views are used by Oracle Enterprise Manager, which is the primary interface for accessing information about system performance. After an instance is started, the V$ views that read from memory are accessible. Views that read data from disk require that the database be mounted, and some require that the database be open.

GV$ Views

For almost every V$ view described in this chapter, Oracle has a corresponding GV$ (global V$) view. In Real Application Clusters, querying a GV$ view retrieves the V$ view information from all qualified instances. In addition to the V$ information, each GV$ view contains an extra column named INST_ID of datatype NUMBER. The INST_ID column displays the instance number from which the associated V$ view information was obtained. The INST_ID column can be used as a filter to retrieve V$ information from a subset of available instances. For example, the following query retrieves the information from the V$LOCK view on instances 2 and 5:

SQL> SELECT * FROM GV$LOCK WHERE INST_ID = 2 OR INST_ID = 5;

V$ACCESS

V$ACCESS displays information about locks that are currently imposed on library cache objects. The locks are imposed to ensure that they are not aged out of the library cache while they are required for SQL execution.

V$ACTIVE_INSTANCES

V$ACTIVE_INSTANCES displays the mapping between instance names and instance numbers for all instances that have the database currently mounted.

V$ACTIVE_SERVICES

V$ACTIVE_SERVICES displays information about the active services in the database.

V$ACTIVE_SESSION_HISTORY

V$ACTIVE_SESSION_HISTORY displays sampled session activity in the database. It contains snapshots of active database sessions taken once a second. A database session is considered active if it was on the CPU or was waiting for an event that didn't belong to the Idle wait class. Refer to the V$EVENT_NAME view for more information on wait classes. This view contains one row for each active session per sample and returns the latest session sample rows first. A majority of the columns describing the session in the active session history are present in the V$SESSION view.