The AWR and ASH Wait Event Tables
Another important area of wait event
statistics is the Oracle Database 10g Active Session History
(ASH) component using the v$active_sess_hist view and the
wrh$active_session_history table. The ASH stores the
history of a recent session’s activity, and facilitates the
analysis of system performance at the current time. ASH is
designed as a rolling buffer in memory, and earlier information
is overwritten when needed. ASH uses the memory of the SGA.
Figure 4.12 - Relationship between v$
views and wrh$ event tables
Another innovation is the
hash key for tracking
session identification. This hash key tracks common session
processes and allows session tracking in cases like OCI session
bouncing where each call to Oracle has a different session ID.
The ASH samples wait events every second
and tracks the waits in the new v$active_sess_hist view.
This view is flushed to the wrh$ tables every hour, or
when a new AWR snapshot is taken. The following list shows the
Oracle Database 10g wrh$ wait event tables.