Waits are timed if and only if the TIMED_STATISTICS parameter is set to TRUE.Let me endorse what others have said before, that the overhead of timed statistics is negligible. If you need to convince yourself, use the SET TIMING ON command in SQL*Plus to measure the elapsed time of a benchmark query. Use an otherwise idle system and take ten or more measurements with and without timed statistics. You will be hard pressed to discern any significant difference.

[@more@]Without timed statistics, Oracle records the reason for each wait before it begins to wait, and when the wait is over, it records whether it timed out. But with timed statistics enabled, Oracle checks the time just before and after each wait, and also records the time waited. The time waited is recorded in hundredths of a second—that is, centiseconds.

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Timed Statistics

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