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Oracle执行计划讲解（一）

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Jun 07, 2016 pm 05:08 PM

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看懂Oracle执行计划是优化的第一步，让我们从下面的例子开始吧。下面为补充内容 1、创建测试表 SQLgt; create ta

看懂Oracle执行计划是优化的第一步，让我们从下面的例子开始吧。

下面为补充内容

1、创建测试表

oracle优化器：RBO和CBO两种，从oracle10g开始优化器已经抛弃了RBO，下面的列子说明CBO大概是怎样的

       现象t表还没有被分析，提示/*+dynamic_sampling(t 0) */*的目的是让CBO无法通过动态采样获取表中的实际数据情况，此时CBO只能根据T表中非常有限的信息（比如表中的extents数量，数据块的数量）来猜测表中的数据。从结果中可以看到CBO猜出表中id=1的有195条，这个数值对于表的总数来说，是一个非常小的值，所以CBO选择了索引而不是全表扫描。
      而实际情况如下所示：        通过动态取样，CBO估算出行数为49454，非常接近于真实50820数目。选择了全表扫描。
       我们来收集一下统计信息

现在扫描过的行数为50815。

如果我们更新了所有的id为99看看。

因为没有对表进行分析，所以表中的分析数据还是之前的信息，CBO并不知道。我们可以看出Rows值为1，也就是说CBO人为表T中的ID=99的值只有1条，，所有选择仍然是索引。

我们收集一把统计信息。

上面为补充内容，下面正式开始

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