RE: Density calculation. Was: Incorrect cardinality estimate
- From: "Laimutis Nedzinskas" <Laimutis.Nedzinskas@xxxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Thu, 25 Jan 2007 14:43:29 -0000
In the other words Oracle is using histogram in the best way it can:
- lower bound of cardinality(frequency) is known only for popular values
(because of consecutive buckets)
- for all the other values Oracle may only guess. It can occupy the
whole bucket or just a fraction of it.
Then density(derived from average frequency adjusted by frequency
deviation as pointed out by Jonathan Lewis) and cleared from popular
values is a decent guess for actual cardinality.
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Wolfgang Breitling
For a height-balanced histogram the (non-null) column values are sorted
and then "filled" into buckets of num_non_null_rows/buckets rounded
(most likely up) to an integer. Then the first value, the highest value
in each bucket, and the last value are recorded.
Popular values are those that occur as the highest value in more than
one (consecutive) bucket.
Fyrirvari/Disclaimer
http://www.landsbanki.is/disclaimer
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