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From "Dag H. Wanvik (JIRA)" <j...@apache.org>
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.
Date Tue, 22 May 2012 15:26:52 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3790?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13281032#comment-13281032
] 

Dag H. Wanvik commented on DERBY-3790:
--------------------------------------

Thanks, Kristian! I have gone though 1b, and it looks good: +1

Small (optional) comments:

- DiposableIndexStatistics#insertData

  * "int reducedRowNumber = ROW_COUNT / 3": Would be nice to have a
  comment why this table receives fewer rows? Arbitrary or not?
  Same for ROW_COUNT really: does it need to be chosen within an
  interval currently for the test to work?

  * Magic constants: 2000, 19, 10?

- DiposableIndexStatistics#createAndPopulateTables
  * space after "+" (several instances):
        Assert.assertTrue(stats.getStatsTable(tbl).length == preFkAddition +1);

- The sleeps in KeepDiposableStatsPropertyTest and
  AutomaticIndexStatisticsTest#testNoUpdateTriggeredBySingleColumnUniqueIndex
  could be removed (future) if we augment the current
  WAIT_FOR_POST_COMMIT method to also wait for the istat daemon to
  complete its thing.

- Call "JDBC.assertDrainResultsHasData(stmt.executeQuery("select
  count(*) from " + TAB))" in KeepDiposableStatsPropertyTest might
  have a comment to say its there to trigger daemon to work.

                
> 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-3790-1c-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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