* v$system_event: This is the high-level view. Like v$sysstat, its contents are reset to zero at instance startup, the values contain summary wait information from all Oracle sessions, and the values increase over time.

* v$session_event: This is just like v$system_event, except it contains the session ID, known as the SID (not the Oracle SID, but the session's session ID). Like v$sesstat, when a session disconnects, the session's wait information is removed; when a session connects, all values begin incrementing from zero.

* v$session_wait: This is real-time session level information. Starting in Oracle Database 10g, you can also get this information in v$session, which can be very useful and more efficient. The value of querying the real-time information is that you can see the lower-level parameters: p1, p2, and p3. These parameters contain very detailed information about the specific wait. For example, if the event is db file scattered read, p1 contains the first block's file number, p2 contains the block number, and p3 contains the number of blocks requested. However, for the event latch free, p2 contains the latch number, which can be cross-referenced in v$latchname.