今天truncate一个分区时明显比以前慢很多，查看进程的等待事件是enq: RO - fast object reuse，以前truncate都很快没有特意看过运行时的等待事件。

百度后找到答案：dbw0进程写回buffer cache中的脏数据块, 出现这个等待事件时运行alter system flush buffer_cache;会有效，我没有运行当我再去看时已经执行完了

据说用dbms_stats进行分析时也可能会遇到

Applies to: 
Oracle Server - Enterprise Edition - Version: 10.2.0.4
This problem can occur on any platform.
 
Symptoms
* Database has been recently upgraded from 10.2.0.1 to 10.2.0.4.
* There is 'enq: RO - fast object reuse' contention when gathering 
  schema/table statistics in parallel using DBMS_STATS package (with DEGREE>1).
 
 
Cause
Bug 7385253 10.2.0.4 RDBMS 10.2.0.4 BUFFER CACHE PRODID-5 PORTID-59
Abstract: DBWR IS CONSUMING HIGH CPU
 
The system state dumps shows that the CKPT background process is the one 
holding the needed RO enqueue although it is actually doing nothing.

ITPub博客

truncate遭遇enq: RO - fast object reuse等待事件

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