#### LATCH ``` sql --根据文件号，块号找到latch select hladdr from x$bh where file#= and dbablk=; --查看此latch保护的buffer selecft file#,dbablk,owner,object_name from x$bh a, dba_objects b where hladdr='' and a.obj=b.data_object_id; --如果两个程序不断访问同一个latch中块，就会出现热链争用 [latch:cache buffers chains] selecft sid,event,p1raw,p2raw from v$session where wait_class<>'Idle' orer by event; --其中，P1raw是latch地址，P2raw是latch编号(v$latch_children的latch#列) --以上争用，可以通过修改隐含参数，增加cache数量 alter system set "_db_block_hash_latches"=32768 scope=spfile; ``` 等待事件 Buffer Busy Waits ,原因大多是DML操作。读不阻塞写，写(独占Buffer Pin锁）会阻塞读。 Oracle修改buffer的步骤 - 申请持有CBC Latch - 获取独占Buffer Pin锁 - 释放CBC Latch - 生成redo数据 - 修改Buffer中的内容 - 申请持有CBC Latch - 释放独占Buffer Pin锁 - 释放CBC Latch