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Oracle物理读与逻辑读

Jun 07, 2016 pm 05:07 PM
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Physical Reads(物理读): 逻辑读内容在内存中,不需要读硬盘。 计算公式可以如下:Physical Reads = db block gets + consist

Physical Reads(物理读):

逻辑读内容在内存中,不需要读硬盘。

计算公式可以如下:Physical Reads = db block gets + consistent gets;


Logical Reads(逻辑读):

物理读是内容不在内存中,要去硬盘中读入内存。


db block gets                    --从buffer cache中读取的block的数量。
consistent gets                --从buffer cache中读取的undo数据的block的数量。

db block gets 是在current mode下读取的block数目。current mode下读取数据是为了保证读取到的数据是当前时间点上最新的数据,这样做的目的一般都是为了DML语句的需求,比如需要更新,自然需要知道最新的数据才行;

consistent gets 是在consistent mode下读取的block数目。consistent mode呢主要是为了保证Oracle数据一致读的特性,一般都是在select情况下发生,读到的数据可能是一个实际存在的block,也有可能需要根据scn信息以及transaction相关信息以及回滚段中数据来构造。


总结:

由于在Oracle中,取数据最后都是从Buffer中取,所以每出现一个physicalreads必然会出现一次 logicalreads,但是这里有一个需要注意的地方,就是当出现一个physicalreads后接着会有一个logicalreads这里,实际上这里只算了1 block!

所以说,如果以这样的公式(Physical Reads = db block gets + consistent gets;)来计算Physical Reads,那么逻辑读就已经包含了物理读了。

不管是db block gets还是consistent gets,都可能出现了physicalreads和logicalreads两种情况,,也即是说,db block gets与consistent gets两者已经构成了一次数据库操作中读取的所有block的总次数了。

因此,logicalreads自然也就可以通过如下公式算的:

logicalreads= (db block gets + consistent gets) - physicalreads。

由此,自然也就得出了cache命中率的公式:
Hit Ratio = (db block gets + consistent gets - physicalreads) / (db block gets + consistent gets) * 100%;

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