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https://issues.apache.org/jira/browse/DERBY-6011?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13555857#comment-13555857
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Dag H. Wanvik commented on DERBY-6011:
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I don't know this code, but reading your comments and looking at code and
patch, the explanation and patch seem reasonable to me.
I think adding a regression tests as you suggest is a good idea, for optimizer
changes it would be especially good to have more regression tests since change
in one location can easily have impact elsewhere due to the nature of the
problem.
It seems that this change wouldn't be harmful in a join context though, if
anything it would inform the optimizer better on the branch cost.
> Derby performs very badly (seems to deadlock and timeout) in very simple
> multi-threaded tests
> ---------------------------------------------------------------------------------------------
>
> Key: DERBY-6011
> URL: https://issues.apache.org/jira/browse/DERBY-6011
> Project: Derby
> Issue Type: Bug
> Affects Versions: 10.7.1.1, 10.8.2.2, 10.9.1.0
> Environment: Lenovo laptop with SSD's, Windows 7, 64-bit, Sun JDK
> 1.6.xx
> Reporter: Karl Wright
> Attachments: derby.log, force-specific-index.diff, manifoldcf.log,
> prefer-unique-index-v1.diff
>
>
> The Apache ManifoldCF project supports Derby as one of its underlying
> databases. Simple tests, however, demonstrate that Derby is apparently
> deadlocking and timing out repeatedly under multi-thread conditions. This
> problem is long-standing, and is not exhibited by any other database
> ManifoldCF supports, and makes a simple test take between 6x and 12x as long.
> There is a trivial test with demonstrates the problem vs. other databases.
> Please do the following (once you have java 1.6+, svn 1.7+, and ant 1.7+
> available):
> (1) Check out https://svn.apache.org/repos/asf/manifoldcf/trunk
> (2) Run the following ant target to download the dependencies: "ant
> make-core-deps"
> (3) Run the Derby test: "ant run-rss-tests-derby" . Note the time required -
> at least 180 seconds, can be up to 360 seconds.
> (4) Run the equivalent HSQLDB test: "ant run-rss-tests-HSQLDB". This test
> takes about 31 seconds to run.
> The output of the Derby test can be found in the directory
> "tests/rss/test-derby-output". Have a look at manifoldcf.log, where all
> long-running queries are reported. Derby.log is also included, which shows
> only that during the test's cleanup phase the database is deleted before it
> is shutdown, which is not pertinent to the performance issue.
> I am available to assist with ManifoldCF, if that seems to be required.
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