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  Oracle Database Tips by Donald Burleson

Using the Wait Interface

So why would anyone want to look at these performance views anyway?  Just like there are some people who like to take things apart to see what's inside, some people just want to know what makes the Oracle database "tick."  However, most are more interested is keeping their bosses and the end users happy. If keeping everyone happy is the goal, this book is a great place to learn how to solve performance problems.  For the clinically curious, this book is also a great place since knowing how to use these views to solve problems is a good starting point for doing further research.

There are several ways to use the Wait Interface to identify the cause for a system slowdown.  One way is using a set of start/stop tables that are created and then dropped when data review is complete.  This way helps reduce the inherent inaccuracies in the Wait Interface in the cumulative numbers gathered by some of the tables.  Refer to the start_system_events.sql, finish_system_events.sql and difference_system_events.sql in Chapter 2. Examples can be found in the case study section of this chapter. 

Another way to use the Wait Interface is to look directly at v$session_wait.  This approach works well when there is a specific user or process that is running slower than usual or slower than is required to meet business requirements.   

* session_waits.sql 

-- *************************************************
-- Copyright 2003 by Rampant TechPress
-- ************************************************* 
/* session_waits.sql */
column SID             format 999
column EVENT           format a28
column P1TEXT          format a15
column P3TEXT          format a8
column P2TEXT          format a8
column SECONDS_IN_WAIT format 99999 heading SECONDS|IN_WAIT
column WAIT_TIME       format 999 heading WAIT|TIME
column STATE           format a18
column P1              format 999999999999
column P2              format 9999
column P3              format 99 
select
   SID,
   EVENT,
   P1TEXT,
   P1,
   P2TEXT,
   P2,
   P3TEXT,
   P3,
   WAIT_TIME,
   SECONDS_IN_WAIT,
   STATE
from
   V$SESSION_WAIT
where
See Code Depot

Note that in versions as recent as 9.2.0.3, this query sometimes returns a high number of rows with "null event."   Do not automatically reject or ignore these rows.  It seems Oracle 9i has a bug that misclassifies some valid wait events as "null event."  Bug # 1743159 was mentioned in Chapter 2, so check MOSC for an updated status of this and other bugs.


The above book excerpt is from:

Oracle Wait Event Tuning

High Performance with Wait Event Iinterface Analysis 

ISBN 0-9745993-7-9  

Stephen Andert 

http://www.rampant-books.com/book_2004_2_wait_tuning.htm

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