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Oracle ASH

Oracle Tips by Burleson Consulting

The Oracle Wait Event Model

The Oracle wait event model was originally introduced in Oracle7 and eventually became a popular tuning method despite the complex and cumbersome wait exposure interfaces.

The Oracle wait event interface evolves with every new version of Oracle database software and provides DBAs with valuable information or insights about where time is consumed by SQL statements, sessions, and the data base as a whole. Wait event information is gathered by the Oracle MMON background process, stored in intermediate x$ structures in a circular SGA buffer, and later transferred to Oracle10g dba_hist  tables.

Prior to Oracle10g, capturing wait event information was a cumbersome process involving the setting of special events like 10046 and the reading of complex trace dumps.

The new Automated Session History  (ASH) tables revolutionized Oracle wait event tuning.  Earlier versions of Oracle provided special views called v$session_wait and v$system_event  that contained detailed information about the wait state for ongoing Oracle transactions.  This section explores how Oracle waits can be analyzed to look for system-wide bottlenecks and individual objects that experience wait issues.

Get your proper ASH license!

The ASH, AWR, and ADDM are separate components of the Oracle Enterprise Manager Diagnostic Pack which must be licensed as a separate option.

Fortunately, Oracle10g has simplified the way that wait event information is captured and there are a wealth of new v$ and wrh$ views relating to Oracle wait events as shown in Figure 16.1.

Figure 16.1: Some common ASH dba_hist views of Oracle event data

Oracle10g has introduced some new wait events, and the 10g database kernel now captures statistics on more than 800 specific wait events. These new wait events are the result of Oracle breaking out their latch waits into their individual components and breaking-out enqueue waits, or locks, into a finer level of granularity.

Donít lose wait data!

Whenever an AWR snapshot is taken, the current contents of the x$ wait structures are transferred to the corresponding dba_hist tables.  Since the wait information is collected in a circular SGA buffer, wait information may be lost if this buffer rolls over between your AWR snapshot interval. 


This is an excerpt from my latest book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts:




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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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