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https://issues.apache.org/jira/browse/DERBY-6156?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13762235#comment-13762235
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Mamta A. Satoor commented on DERBY-6156:
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I am looking at this issue. DERBY-6045 found the issue to be in estimateCost
method. To start with, I will look at various subclasses of Optimizable to see
if we rely on the statistics in those subclasses. Additionally, I will also
search for TableDescriptor.statisticsExist method to see what classes rely on
this call and if we need to account for missing stats for single-colunm unique
indexes.
> 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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