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From "Mike Matrigali (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-269) Provide some way to update index cardinality statistics (e.g. reimplement update statistics)
Date Fri, 18 Jul 2008 16:48:31 GMT

    [ https://issues.apache.org/jira/browse/DERBY-269?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12614785#action_12614785
] 

Mike Matrigali commented on DERBY-269:
--------------------------------------

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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