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From "Kristian Waagan (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-4849) Re-compilation may cause duplicate entries in the XPLAIN table
Date Tue, 02 Nov 2010 07:48:24 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4849?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12927323#action_12927323

Kristian Waagan commented on DERBY-4849:

I'm investigating this issue, and found that it is the insert of a new entry that conflicts
with the lookup of statistics.

Changing to read uncommitted is an option, but it looks like the code in the relevant code
path (TableDescriptor and DataDictionaryImpl) isn't set up to return multiple rows when using
read uncommitted. I have talked to Dag about this, which made some changes in that area of
the code some years ago, and he believes the restriction was put there to be defensive (I
think up until then all access was done using repeatable read).

A potentially simpler fix could be to access the statistics system table with the NO_WAIT
flag and retry if the lock cannot be obtained immediately. Does anyone see any notable drawbacks
with this approach?
I'll disable the test if it is clear that my efforts today won't be sufficient to fix it.

> Re-compilation may cause duplicate entries in the XPLAIN table
> --------------------------------------------------------------
>                 Key: DERBY-4849
>                 URL: https://issues.apache.org/jira/browse/DERBY-4849
>             Project: Derby
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions:,
>            Reporter: Kristian Waagan
>            Assignee: Kristian Waagan
>            Priority: Minor
>         Attachments: derby-4849-1a-narrow_fix.diff, derby-4849-2a-broad_fix.diff, derby-4849-2b-broad_fix_with_test.diff,
derby-4849-2b-broad_fix_with_test.stat, derby-4849-2c-broad_fix_with_test.diff, derby-4849-xplain_duplicate_stacktrace.txt
> If happening at the right moment, a re-compilation request may cause duplicate entries
in the XPLAIN statement tables.
> I have only confirmed this for the SYSXPLAIN_STATEMENTS table, and I do not know if the
other XPLAIN tables are affected.
> The error is highly intermittent, and so far I have only been able to trigger it when
testing the automatic index statistics update prototype.
> See the attached stack-trace for some more details.

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