FYI: In 10.2.0.4 cardinality estimation change

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


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