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Date: Tue, 22 May 2012 11:20:41 +0000 (UTC)
From: "Knut Anders Hatlen (JIRA)" <jira@apache.org>
To: derby-dev@db.apache.org
Message-ID: <1984787982.7476.1337685641271.JavaMail.jiratomcat@issues-vm>
Subject: [jira] [Commented] (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.
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    [ https://issues.apache.org/jira/browse/DERBY-3790?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13280883#comment-13280883 ] 

Knut Anders Hatlen commented on DERBY-3790:
-------------------------------------------

You might want to update the class name in KeepDisposableStatsPropertyTest.java's license header too.
                
> 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
>            Assignee: Kristian Waagan
>         Attachments: derby-3790-1a-skip_stats_scui.diff, derby-3790-1b-skip_stats_scui.diff
>
>
> 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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