当一个数据块读入sga区，相应的buffer header会被放置到hash列表上,我们称其这hash chains,chain在中文的意为链条或串的意思，表达就是关连性.如果一个进程想访问或修改hash chain上的block,它首先要获得”cache buffers chains” latch。
原因一：低效率的SQL语句（主要体现在逻辑读过高）
cache buffers chains latch很大程度与逻辑读有关，所以要观注v$sql中BUFFER_GETS/EXECUTIONS大的语句。
同时每一个逻辑读需要一个latch get 操作及一个cpu操作，这样的sql也会很耗cpu资源。
原因二：热块（访问过于频繁）
找出热点块方法一：
--找出p1raw select p1,p1raw from v$session_wait where event='latch: cache buffers chains'; --找到对象 SELECT /*+ RULE */ E.OWNER || '.' || E.SEGMENT_NAME SEGMENT_NAME, E.PARTITION_NAME, E.EXTENT_ID EXTENT#, X.DBABLK - E.BLOCK_ID + 1 BLOCK#, X.TCH, L.CHILD# FROM SYS.V$LATCH_CHILDREN L, SYS.X$BH X, SYS.DBA_EXTENTS E WHERE X.HLADDR = '00000002576EE018'--p1raw AND E.FILE_ID = X.FILE# AND X.HLADDR = L.ADDR AND X.DBABLK BETWEEN E.BLOCK_ID AND E.BLOCK_ID + E.BLOCKS - 1 ORDER BY X.TCH DESC;
找出热点块方法二：
--直接找出热点块
SELECT OBJECT_NAME, SUBOBJECT_NAME
FROM DBA_OBJECTS
WHERE DATA_OBJECT_ID IN
(SELECT DATA_OBJECT_ID
FROM (SELECT OBJ DATA_OBJECT_ID, FILE#, DBABLK, CLASS, STATE, TCH
FROM X$BH
WHERE HLADDR IN (SELECT ADDR
FROM (SELECT ADDR
FROM V$LATCH_CHILDREN
ORDER BY (GETS + MISSES + SLEEPS) DESC)
WHERE ROWNUM < 10)
ORDER BY TCH DESC)
WHERE ROWNUM < 10);
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