Réf. : histograms, buckets, and bind variables
- From: huy-luan.le@xxxxxxxxxxxx
- To: oracle-l@xxxxxxxxxxxxx
- Date: Wed, 17 Nov 2004 17:27:25 +0100
Wolfgang Breitling said :
A commonly held belief is that histograms are useless, i.e. have no effect
on the access plan, if bind variables are used since the value is not known
at parse time and the CBO histograms are only ever used by the CBO can not
determine from the histogram if it should use an available index or not.
While the latter is true, the gathering of histograms still can change the
access plan. Why and how? Because
a) the optimizer uses the density in its filter factor calculation, not
NDV.
b) the density is calculated differently for columns with histograms, not
simply as 1/NDV
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