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https://issues.apache.org/jira/browse/DERBY-269?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12614785#action_12614785
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Mike Matrigali commented on DERBY-269:
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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.
> Provide some way to update index cardinality statistics (e.g. reimplement
> update statistics)
> --------------------------------------------------------------------------------------------
>
> Key: DERBY-269
> URL: https://issues.apache.org/jira/browse/DERBY-269
> Project: Derby
> Issue Type: New Feature
> Components: SQL
> Affects Versions: 10.0.2.0, 10.0.2.1, 10.1.1.0, 10.2.2.0, 10.3.1.4
> Reporter: Stan Bradbury
> Assignee: Mamta A. Satoor
> Attachments: DERBY269_update_statistics_svndiff_ver1.txt,
> DERBY269_update_statistics_svndiff_ver2.txt,
> DERBY269_update_statistics_svnstat_ver1.txt,
> DERBY269_update_statistics_svnstat_ver2.txt
>
>
> Performance problems are being reported that can be resolved by updating the
> cardinality statistics used by the optimizer. Currently the only time the
> statistics are guaranteed to be an up-to-date is when the index is first
> created on a fully populated table. This is most easily accomplished on an
> existing table by using the command:
> alter table <table-name> compress [sequential]
> Compress table is an I/O intensive task. A better way to achieve this would
> be to re-enable parser support for the 'update statistics' command or
> re-implement the update in some other fashion.
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