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From "Mamta A. Satoor (JIRA)" <j...@apache.org>
Subject [jira] [Created] (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, 09 Apr 2013 00:22:16 GMT
Mamta A. Satoor created DERBY-6156:
--------------------------------------

             Summary: 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


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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