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https://issues.apache.org/jira/browse/DERBY-4881?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12927386#action_12927386
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Knut Anders Hatlen commented on DERBY-4881:
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The patch looks good to me. And it removes more code than it adds! :) It looks
like Dag handled most of the edges for the read uncommitted case in DERBY-3678,
and with your change to ignore td==null, I think it should be safe in the
list!=null case too.
I was able to reproduce the deadlock in XplainStatisticsTest on nearly every
run with an insane build in my environment. The patch seems to have fixed that,
and now I'm not able to reproduce it.
+1 to commit.
> Deadlock accessing SYS.SYSSTATISTICS
> ------------------------------------
>
> Key: DERBY-4881
> URL: https://issues.apache.org/jira/browse/DERBY-4881
> Project: Derby
> Issue Type: Bug
> Components: SQL
> Affects Versions: 10.3.3.0, 10.4.2.0, 10.5.3.0, 10.6.2.1, 10.7.1.0
> Reporter: Kristian Waagan
> Assignee: Kristian Waagan
> Priority: Minor
> Attachments: derby-4881-1a-deadlock_fix.diff
>
>
> Transactions accessing index statistics can deadlock if one of them inserts
> new entries and the other selects from the system table. Inserts happens for
> instance when update of index statistics are perform manually, or when a
> table is compressed (given that the table has indexes and contains some
> rows). This issue may be more problematic when automatic update of index
> statistics is implemented.
> Issue discovered when writing a regression tests for DERBY-4849, see
> discussion there. The bug is timing dependent, but has been observed on a
> variety of JVMs and platform architectures.
> To sum up:
> o using NO_WAIT + retry was suggested, but turned out to be an infeasible
> solution
> o current approach is to allow using read uncommitted isolation level when
> accessing statistics in the system table (take no locks)
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