The dynamic performance data dictionary views are colloquially referred to as the V$ and GV$ views. These views are constantly updated by Oracle and reflect the current condition of the instance and database. Dynamic views are critical for diagnosing real-time performance issues.

The V$ and GV$ views are indirectly based on underlying X$ tables, which are internal memory structures that are instantiated when you start your Oracle instance. Some of the V$ views are available the moment the Oracle instance is started.

For example, V$PARAMETER contains meaningful data after the STARTUP NOMOUNT command has been issued and does not require the database to be mounted or open. Other dynamic views (such as V$CONTROLFILE) depend on information in the control file and therefore contain significant information only after the database has been mounted.

Some V$ views (such as V$DB) provide kernel-processing information and thus have useful results only after the database has been opened.

At the top layer, the V$ views are actually synonyms that point to underlying SYS.V_$ views. At the next layer down, the SYS.V_$ objects are views created on top of another layer of SYS.V$ views. The SYS.V$ views in turn are based on the SYS.GV$ views. At the bottom layer, the SYS.GV$ views are based on the X$ memory structures.

The top-level V$ synonyms and SYS.V_$ views are created when you run the catalog.sql script, which you usually do after the database is initially created. Figure 10-2 shows the process for creating the V$ dynamic performance views.

Figure10-2.CreatingtheV$dynamicperformancedatadictionaryviews

Accessing the V$ views through the topmost synonyms is usually adequate for dynamic performance information needs. On rare occasions, you will want to query internal information that may not be available through the V$ views. In these situations, it is critical to understand the X$ underpinnings.

If you work with Oracle Real Application Clusters (RACs), you should be familiar with the GV$ global views. These views provide global dynamic performance information regarding all instances in a cluster (whereas the V$ views are instance specific). The GV$ views contain an INST_ID column for identifying specific instances in a clustered environment.

You can display the V$ and GV$ view definitions by querying the VIEW_DEFINITION column of the V$FIXED_VIEW_DEFINITION view. For instance, this query displays the definition of the V$CONTROLFILE:

SQL> select view_definition from v$fixed_view_definition where view_ name=’V$CONTROLFILE’; VIEW_DEFINITION

select STATUS , NAME, IS_RECOVERY_DEST_FILE, BLOCK_SIZE, FILE_SIZE_ BLKS,   CON_ID from GV$CONTROLFILE where inst_id = USERENV(‘Instance’)