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：

全文请参考原链接地址： about consistent read gets