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From "Mamta A. Satoor (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-6156) Starting Derby 10.9, we do not not keep statistics for single-column unique index. Check the code to see optimizer calculates correct cost even with the missing stats for such indexes
Date Tue, 10 Sep 2013 23:07:53 GMT

    [ https://issues.apache.org/jira/browse/DERBY-6156?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13763667#comment-13763667
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Mamta A. Satoor commented on DERBY-6156:
----------------------------------------

I have looked at the optimizer related code in estimateCost of various Optimizable sub-classes
and other than FromBaseTable, I do not see any other code using statistics info. Additionally
PredicateList's selectivity method looks at statistics but the call to this method is from
FromBaseTable:estimateCost. So, based on this, I do not believe there are other places in
the code where we need to make any changes for missing stats for single column unique index.
                
> Starting Derby 10.9, we do not not keep statistics for single-column unique index. Check
the code to see optimizer calculates correct cost even with the missing stats for such indexes
> ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: DERBY-6156
>                 URL: https://issues.apache.org/jira/browse/DERBY-6156
>             Project: Derby
>          Issue Type: Task
>          Components: SQL, Store
>            Reporter: Mamta A. Satoor
>              Labels: derby_triage10_11
>
> With DERBY-3790(Investigate if request for update statistics can be skipped for certain
kind of indexes, one instance may be unique indexes based on one column.) we have stopped
keeping statistics for single column unique index. But with DERBY-6045(in list multi-probe
by primary key not chosen on tables with >256 rows), we found that optimizer code was not
adjusted to tackle the missing stats and hence we were not picking index scan even though
the single column unique index was the right choice. DERBY-6045 has been fixed but we should
make sure there are no other places in the optimizer code where we have missed code changes
to account for missing statistics for single-column unique indexes.

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