今天在网上看到有人在讨论预读和物理读的区别,个人觉得物理读和预读都是IO操作,都是需要从磁盘中读取数据到内存,只是读取的时间有所不同,之后查了一下MSDN确认了这种想法。 SET STATISTICS IO ON SELECT COUNT(*) FROM LINEITEM Table 'LINEITEM'. Scan count 3, logical reads 22328, physicalreads 3, read-ahead reads20331, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
重复执行这条语句看到预读已经变为0,因为所有的数据已经在缓存中了: Table 'LINEITEM'. Scan count 3, logical reads 22328, physicalreads 0, read-ahead reads 0,lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
对于预读的解释: |