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.
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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.
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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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