Prerequisites to Collecting SQL Trace Data

There are a couple prerequisite parameters that must be addressed no matter which method is used for enabling tracing. Timed statistics must be enabled in order to get timing information about the process. Timed statistics can be enabled at startup using the pfile or spfile. In the pfile, set the parameter to timed_statistics = TRUE, while the spfile could be updated with the following command:

ALTER SYSTEM SET TIMED_STATISTICS = TRUE SCOPE=BOTH;

In the early days of Oracle, possibly as recently as version 7, using this parameter had a negative impact, reportedly around 10%, on database performance. However, since version 8 and definitely in 9i and 10g, if there is any performance impact, it is widely believed to not be measurable.

In spite of this, Metalink documentation still appears to be divided on this issue. The reference note for this parameter states, “Normally, timed_statistics should be false.” Meanwhile note 30824.1, states that the small amount of overhead from tuning this tracing on is worth the benefits gained.

In the face of this ambiguity from Oracle, many Remote DBAs choose to leave this to TRUE on an instance-wide level for most databases. The exception to this is when a bug is impacting normal operations and cannot be patched.

It can also be enabled with either an ALTER SYSTEM or ALTER SESSION call at any time for either a specific session or the whole instance. There are a few ways to accomplish this, but only two ways are shown here. These should work for most installations.

The above book excerpt is from:

Oracle Wait Event Tuning

High Performance with Wait Event Interface Analysis

ISBN 0-9745993-7-9

Stephen Andert

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