Probably the most important fixed view for the performance analyst is V$SQL. This view shows several important attributes of the SQL statements whose header information currently reside in the shared pool.With V$SQL, you can rank SQL statements in your system by the amount of work they do, or by whatever measure of efficiency you like.You can even sense the presence of how distinct SQL texts might have been able to make more effective use of bind variables.
V$SESS_IO is a simple fixed view that allows you to measure the logical and so-called physical I/O that has been generated for a session.
Each row in V$SYSSTAT contains an instance-wide statistic. Most statistics are tallies of operations that have occurred since the most recent instance startup. V$SYSSTAT rows are subject to overflow errors.
The denormalized structure of V$SYSSTAT makes it easy to find out what the system has done since the most recent instance startup, without having to do a join.
the system-wide scope is probably the incorrect action scope for your diagnostic data collection. V$SESSTAT contains the same statistics as V$SYSSTAT, except at the session level.
The V$SYSTEM_EVENT fixed view records aggregated statistics about instrumented code paths that the Oracle kernel has executed since its most recent instance startup.
the system-wide scope is often the incorrect scope for diagnostic data collection. V$SESSION_EVENT
provides the ability to collect properly session-scoped diagnostic data for Oracle kernel code paths
Unlike the V$SYSTEM_EVENT and V$SESSION_EVENT fixed views, V$SESSION_WAIT does not contain an aggregation of historical events. Instead, it provides a view into what a specified session is doing right now
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