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https://issues.apache.org/jira/browse/DERBY-4849?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12927343#action_12927343
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Kristian Waagan commented on DERBY-4849:
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Thanks for the feedback, Knut.
As for the NO_WAIT + retry option, the statistics in this case are accessed in
a nested transaction which could be rollback back. However, I found it hard to
guarantee that the relevant methods will always be called in a nested
transaction, as they are used in a few more places.
For clarity, I was thinking of using this option for the case when we are
reading the statistics, not when we are inserting/deleting entries. This was
based on the assumption that almost all access to the system statistics table
will be reads.
I'll pursue the approach of modifying the data dictionary code to allow
returning more than one row with isolation read uncommitted (this is of course
allowed in the general case, but the data dictionary code seems to avoid it).
It is not clear to me which issues we are exposed to, if any, by doing that.
> 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, 10.7.1.0
> 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-4849-2c-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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