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.

当且仅当TIMED_STATISTICS 被设置为TRUE, waits 事务才会被纪录时间，我同意其他人以前所說的, 因為TIMED_STATISTICS的設置而產生的負荷可以忽略不計, 如果你需要去說服自己, 那麼在SQL*Plus中使用set timing on 命令去測試一個基準的查詢所消耗的時間, 用另外一個閒置的系統 , 然後分別使用或不使用timed_statistics 來做10次或以上的測量 . 你將很難看到任何的重大不同.

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.

沒有使用timed statistics , Oracle在事務開始wait以前紀錄每個wait的原因, 當wait結束, Oracle會紀錄它們是否超時, 但是如果timed_statistics 設置為ture , Oracle會在,每個wait開始及結束的時候check時間, 也會紀錄下時間, 紀錄時間以 1/100秒計算 .

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TIMED_STATISTICS parameter

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