Oracle CR块

Oracle中的CR块是用来维护读一致性的数据块。当查询某些数据时，如果发现数据块的版本比我们要查询的数据新，就需要构建CR块。构建CR块的过程是先从UNDO中查找这个数据的改前映像（PRE IMAGE），然后将改前映像和当前的数据块合并，从而得到一致性的数据。<span class="em">1</span><span class="em">2</span><span class="em">3</span>
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