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  Oracle Tips by Burleson

Easy Oracle Automation
Chapter 4 -
Automatic Workload Repository and Automatic Database Diagnostic Monitor

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.                

  • wrh$_active_session_history

  • wrh$_active_session_history_bl

  • wrh$_bg_event_summary  


The above text is an excerpt from:

Easy Oracle Automation
Oracle10g Automatic Storage, Memory and Diagnostic Features

ISBN 0-9745993-6-0

by Dr. Arun Kumar R.

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