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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-4849) Re-compilation may cause duplicate entries in the XPLAIN table
Date Tue, 19 Oct 2010 13:37:28 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4849?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12922556#action_12922556
] 

Knut Anders Hatlen commented on DERBY-4849:
-------------------------------------------

Thanks Kristian. The fix looks good to me. I also ran the test six times without the fix,
and it failed twice.

There's one small issue with the test that I think could lead to intermittent failures. The
main JUnit thread runs a wait loop while the helper threads are running, but when it terminates
the loop and checks if everything went well, it doesn't really know that the helper threads
have completed. If they haven't (for example because one of them used a little longer than
expected to start up, or used a little longer in one of the iterations), the asserts in the
test may fail.

I added Thread.sleep(1500) at the beginning of MTSimpleSelect.run() to test this, and then
I got a mismatch in the final assertSingleValueResultSet() -- expected: >277<, found:
>278<. I think we can avoid this problem by calling Thread.join() on the two helper
threads instead of running the wait loop.

> 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: 10.6.2.1, 10.7.0.0
>            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-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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