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https://issues.apache.org/jira/browse/DERBY-3790?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12831384#action_12831384
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martin commented on DERBY-3790:
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comment from Mike Matrigali on #DERBY-3937:
> The current implementations of indexes ... do not maintain an exact count of
> rows.
> Investigate if request for update statistics can be skipped for certain kind
> of indexes, one instance may be unique indexes based on one column.
> ------------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: DERBY-3790
> URL: https://issues.apache.org/jira/browse/DERBY-3790
> Project: Derby
> Issue Type: Improvement
> Components: Store
> Affects Versions: 10.5.1.1
> 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.
> **************************
> **************************
> 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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