Re: FYI: In 10.2.0.4 cardinality estimation change
- From: "조동욱" <ukja.dion@xxxxxxxxx>
- To: riku.rasanen@xxxxxxxxxxxxxxxx
- Date: Fri, 25 Apr 2008 09:33:35 +0900
Thanks for the great info.
I would like to make a sound test case for it.
Dion Cho
2008/4/25, Riku Räsänen <riku.rasanen@xxxxxxxxxxxxxxxx>:
>
> Found interesting change, included in 10.2.0.4.
>
> From Note:5483301.8
>
> Quote:
>
> "If a column has a frequency histogram and a query uses an equality
> predicate over the column looking for a value that does not exists
> in the histogram then the CBO was estimating a cardinality of 1.
> This could favor Nested Loops too much.
>
> The fix introduces a notable change in behavior: instead of estimating
> a cardinality of 1 then with this fix CBO estimates a cardinality of
> (0.5 * the lowest cardinality found in the histogram). This can result
> in plan changes."
>
>
>
> This issue has bitten me several times, mostly because of unnecessary
> histograms. Remains to be seen, if this really affects anything.
>
>
> --
> Riku Räsänen
> Kantamestarit Oy
> www.kantamestarit.fi
>
>
>
> --
> //www.freelists.org/webpage/oracle-l
>
>
>
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