Oracle中的v$和gv$

Oracle contains a set of underlying views that are maintained by the database server and accessible to the database administrator user SYS. These views are called dynamic performance views because they are continuously updated while a database is open and in use, and their contents relate primarily to performance.

Although these views appear to be regular database tables, they are not. These views provide data on internal disk structures and memory structures. You can select from these views, but you can never update or alter them.

Note:

• You can query the dynamic performance views to extract information from them. However, only simple queries are supported. If sorts, joins, GROUP BY clauses and the like are needed, then you should copy the information from each V$ view into a table (for example, using a CREATE TABLE ... AS SELECT statement), and then query from those tables.

• Because the information in the V$ views is dynamic, read consistency is not guaranteed for SELECT operations on these views.

The catalog.sql script contains definitions of the views and public synonyms for the dynamic performance views. You must run catalog.sql to create these views and synonyms. After installation, only user SYS or anyone with SYSDBA role has access to the dynamic performance tables.

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;

概括一下:
gv$和v$都是视图, 它们分别是gv_$和v_$的同义词, ?/rdbms/admin/catalog.sql负责创建这些视图. 对于视图,
仅仅能做简单的查询. X$是所有的数据的来源, 是真正的系统表. 它们全部属于SYS用户.
gv$和v$的区别是多了一列inst_id. 所以gv$视图可以查到RAC的相关信息.

SQL> select instance_name from v$instance;

INSTANCE_NAME
----------------
orcldb1

SQL> select instance_name from gv$instance;

INSTANCE_NAME
----------------
orcldb1
orcldb2