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From "Mamta A. Satoor (JIRA)" <j...@apache.org>
Subject [jira] Created: (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.
Date Mon, 21 Jul 2008 18:49:31 GMT
Investigate if request for update statistics can be skipped for certain kind of indexes, one
instance may be unique indexes based on one column.
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                 Key: DERBY-3790
                 URL: https://issues.apache.org/jira/browse/DERBY-3790
             Project: Derby
          Issue Type: Improvement
          Components: Performance
    Affects Versions: 10.5.0.0
            Reporter: Mamta A. Satoor


DERBY-269 provided a manual way to update the statisitcs. There was some discussion in that
jira entry for possibly optimizing the cases where there is no need to update the statistics.
I will enter the related comments from that jira entry here for reference.

**************************
Knut Anders Hatlen - 18/Jul/08 12:39 AM 
If I have understood correctly, unique indexes always have up to date cardinality statistics
because cardinality == row count. If that's the case, one possible optimization is to skip
the unique indexes when SYSCS_UPDATE_STATISTICS is called. 
**************************


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Mike Matrigali - 18/Jul/08 09:48 AM 
is the cardinality of a unique index 1 or is it row count? 

It is also more complicated than just skipping unique indexes, it depends on the number of
columns in the index because 
in a multi-column index, multiple cardinalities are calculated. So for instance on an index
on columns A,B,C there are 
actually 3 cardinalities calculated: 
A 
A,B 
A,B,C 

I agree that the calculation of cardinality of A,B,C could/should be short circuited for a
unique index. 
**************************



**************************
Knut Anders Hatlen - 18/Jul/08 03:25 PM 
Mike, 
It looks to me as if the cardinality is the number of unique values, so I think the cardinality
of a unique index is equal to its row count (for the full key, that is). You're right that
we can't short circuit it if we have a multi-column index. I don't know if it's worth the
extra complexity to short circuit the A,B,C case, since we'd have to scan the entire index
anyway. For a single-column unique index it sounds like a good idea, though. 
**************************


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