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https://issues.apache.org/jira/browse/DERBY-4121?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12689873#action_12689873
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Knut Anders Hatlen commented on DERBY-4121:
-------------------------------------------
My understanding of the code is that we store the number of unique values, not
the average number of duplicates. The selectivity is calculated this way (from
StatisticsImpl.java):
public double selectivity(Object[] predicates)
{
if (numRows == 0.0)
return 0.1;
/* xxxSTATresolve: for small values of numRows, should we do
something
* special?
*/
return (double)(1/(double)numUnique);
}
If the number of rows is doubled, and the average number of duplicates per
value doesn't change, this method will return a selectivity that is two times
as high as it should be if we don't update the statistics.
> Documentation: more UPDATE_STATISTICS fixes needed for Reference Manual and
> Tuning Derby
> ----------------------------------------------------------------------------------------
>
> Key: DERBY-4121
> URL: https://issues.apache.org/jira/browse/DERBY-4121
> Project: Derby
> Issue Type: Bug
> Components: Documentation
> Affects Versions: 10.5.0.0
> Reporter: Kim Haase
> Priority: Minor
>
> Kathey Marsden comments on DERBY-3787:
> Not a show stopper for the release, but in beginning my buddy testing,
> I noticed the examples should use CALL e.g.
> CALL SYSCS_UTIL.SYSCS_UPDATE_STATISTICS('SAMP','EMPLOYEE','PAY_DESC');
> CALL SYSCS_UTIL.SYSCS_UPDATE_STATISTICS('SAMP', 'EMPLOYEE', null);
> Also in the Tuning Guide under Working with Cardinality Statistics -> When
> Cardinality Statistics Go Stale we should refer users to the
> SYSCS_UTIL.SYSCS_UPDATE_STATISTICS stored procedure to update their
> statistics and cross reference the reference guide.
> Myrna van Lunteren suggests a new issue for this, so I'm filing it.
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