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段上的等待事件 enq: HW

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Jun 07, 2016 pm 05:20 PM

众所周知，Oracle高水位线标志着该线以下的block均被Oracle格式过，通俗一点讲就是该高水位线以下的block都被Oracle使用过。通常

为防止多个进程同时修改HWM而提供的锁称为HW锁。想要移动HWM的进程必须获得HW锁。若在获取HW锁过程中发生争用，则等待enq: HW - contention事件。HW锁争用大部分是大量执行insert所引发的。

众所周知，，Oracle高水位线标志着该线以下的block均被Oracle格式过，通俗一点讲就是该高水位线以下的block都被Oracle使用过。通常在执行insert操作时，当高水位线以下block不够用时，Oracle将会推进高水位线。更进一步讲，当有多个进程在同时进行insert操作时，比较容易引起高水位线争用，主要表现为enq: HW - contention。

如何找到事件：'enq: HW - contention' 热点对象：

查看v$session_wait，应该会有如下等待事件：

7 rows selected通过P3进行DBMS_UTILITY转换可以获知发生争用的文件和block：进而通过file#和block#定位对象：

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