Oracle执行计划 讲解（一）
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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