about consistent read gets

关于consistent read(简称CR),有网友在问这个问题，虽然看似很容易理解，然而我想不见得大家都明白，在我
查询官方文档的过程中，我也发现我以前的认识是错误的或者说理解不完整，下面是涉及到的几个概念，来自官
方文档：

session logical reads :    The sum of "db block gets" plus "consistent gets". This includes logical reads of database    blocks from either the buffer cache or process private memory. cleanouts and rollbacks - consistent read gets:    Number of consistent gets that require both block rollbacks and block cleanouts. cleanouts only - consistent read gets:    Number of consistent gets that require only block cleanouts, no rollbacks. consistent gets:    Number of times a consistent read was requested for a block.  consistent changes:    Number of times a user process has applied rollback entries to perform a consistent read on the block CR blocks created:    Number of CURRENT blocks cloned to create CR (consistent read) blocks. The most common reason for cloning    is that the buffer is held in a incompatible mode. db block gets：   Number of times a CURRENT block was requested

关于CR read相关的几个我都列出来了，上面信息都来自官方文档，描述是非常准确的。

前面我之所以把逻辑读也列出来了，其实是因为很多朋友有一个错误的认识，例如下面这个执行计划：

 

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