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From "Bryan Pendleton (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-4849) Re-compilation may cause duplicate entries in the XPLAIN table
Date Tue, 19 Oct 2010 04:48:27 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4849?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12922434#action_12922434

Bryan Pendleton commented on DERBY-4849:

Perhaps the method name could be getStatementNeedsRecompile()? Just a thought.

Regarding the question of how the XPLAIN tables should behave when the
statement has an error, I don't recall that being discussed in any detail when
we were building that feature.

My quick reaction is that other types of execution failures will suffer from similar
problems: the runtime statistics information will be at best partial and inaccurate,
and at worst will cause crashes when we try to access it.

I think the system would be more robust if it only captured XPLAIN statistics
on successful statement execution. At some later time, when we have more
experience with and confidence in the XPLAIN table feature, we could then
open it up to capturing statistics from failed executions.

> 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
>            Priority: Minor
>         Attachments: derby-4849-1a-narrow_fix.diff, derby-4849-2a-broad_fix.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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