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  Oracle Database Tips by Donald Burleson

Processed iostat data

The processed results can be reviewed and compared to previous days.  This will help determine whether the current performance issue is related to I/O performance.  If so, this might mean I/O is causing the problem and the proper course of action is to determine if any storage configurations have been changed.  More likely, there was a change to the application or how the users are using it and suddenly there are full table-scans where there used to be index reads.  This could be overtaxing some component of the storage system.

Tuning SQL

When looking at a whole-system tuning approach, SQL analysis and tuning may be necessary.  Whether SQL tuning in analyzing a performance issue is a factor will depend on the applications the organization is using and the experience and skill of the developers. Even with talented developers, their goal is to provide a working application. Therefore, tuning can take a back seat until the production users get to assess the scalability of the application and the DBA gets involved because "the database is slow."   

"I wonder if I/O would be improved if we plugged these back in."

SQL tuning becomes a requirement when, after determining that the biggest bottleneck in the system is I/O and that the response time is lacking and impacting the users, it becomes apparent that enhancing the storage hardware is not the only solution.  If the only change in system statistics is that the number of I/O operations has increased dramatically, and there is no corresponding business driver; i.e., acquisition, merger or sales spike, to account for this, there may be a new piece of code or old statistics that are causing unnecessary I/O operations. 

The Oracle Optimizer is used to determine the best way to access the data.  It works in two modes, Rule and Cost.  Rule-based Optimization (RBO) has a set of rules it follows to determine the best execution plan.  The Cost Based Optimizer (CBO) determines the best way to access the data based on the relative cost of different execution plans.   

Before version 8i, the CBO was not always as smart as an average DBA, so hints were often needed.  Hints provided the DBA a way to influence the optimizer toward more efficient execution plans.  Gradually, the CBO has gotten more efficient to the point where now a hint rarely results in a better execution plan.

If it is determined that there is some 'bad' SQL that needs to be tuned, there are several possible target areas:

  • Reduce or eliminate unnecessary I/O, which is commonly done by eliminating a full table-scan. Sometimes the most efficient execution plan is to use a full table-scan, so elimination of full table-scans may not be recommended. 

  • Appropriate index creation and usage is important.  If there is an index that can never be used effectively, then it should be dropped.  If there are several indexes, it might make sense to combine them into a single concatenated index.

  • Use the buffer cache when I/O is appropriate and required.  If there are several small tables that are frequently accessed, and they do not all fit in the buffer cache, increase it if they are the source of a performance problem.

  • Reduce excessive parsing.  This is not necessarily a SQL tuning issue, but if a PL/SQL loop is designed poorly, parsing could result every time through a loop when it is not needed.

There are other factors involved in tuning SQL.  Since there are already good books on the subject, this book will not go into a great deal of detail in this area.  This book will focus on identifying 'bad' SQL with the Wait Interface when that is the cause of a performance problem in the database.

The above book excerpt is from:

Oracle Wait Event Tuning

High Performance with Wait Event Iinterface Analysis 

ISBN 0-9745993-7-9  

Stephen Andert

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