DBA Tips Archive for Oracle

  


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by Michael New, MichaelNew@earthlink.net, Gradation LLC


To activate time measurements in Oracle, the DBA will need to turn on timed_statistics parameter either during database startup via the init.ora file or using an ALTER SYSTEM common.

Starting with Oracle9i, Oracle now supports capturing certain performance data with microsecond (µs) granularity. Views that include microsecond timing include the following.

Performance Views with Microsecond (µs) Granularity
View Name Column Name
V$SYSTEM_EVENT TIME_WAITED_MICRO
V$SESSION_EVENT TIME_WAITED_MICRO
V$SQL CPU_TIME
ELAPSED_TIME
V$LATCH WAIT_TIME
V$SQL_WORKAREA ACTIVE_TIME
V$SQL_WORKAREA_ACTIVE ACTIVE_TIME

Existing columns in other views continue to use centisecond times.

Because microsecond timing might not be appropriate for rolled-up data, STATSPACK displays most times in seconds. It displays average times in milliseconds for easier comparison with operating system monitoring utilities, which often report timing in milliseconds.

For clarity, the time units used are specified in the column headings of each timed column in the Statspack report. The following conventions are used.

If timings are available, the STATSPACK report orders wait events by time. These are listed in the Top-5 and background and foreground wait events sections.

If TIMED_STATISTICS is FALSE for the instance, but a subset of users or programs set TIMED_STATISTICS to TRUE dynamically, then the STATSPACK report output can look inconsistent, where some events have timings (those which the individual programs/users waited for) and some do not. The Top-5 section also looks unusual in this situation.

Optimally, TIMED_STATISTICS should be set to TRUE at the instance level, for ease of diagnosing performance problems.



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Last modified on
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