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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 Thu, 14 Oct 2010 10:07:35 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4849?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12920912#action_12920912
] 

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

I think I see what's going on.

BasicNoPutResultSetImpl.open() discovers that the statement has been invalidated, but only
after the result set has actually been opened. It raises a LANG_STATEMENT_NEEDS_RECOMPILE
exception to notice the caller that the result set is based on an invalid plan. The exception
is caught and handled by this code in GenericPreparedStatement.executeStmt():

			} catch (StandardException se) {
				/* Cann't handle recompiling SPS action recompile here */
				if (!se.getMessageId().equals(SQLState.LANG_STATEMENT_NEEDS_RECOMPILE)
						 || spsAction)
					throw se;
				statementContext.cleanupOnError(se);
				continue recompileOutOfDatePlan;

			}

The call to statementContext.cleanupOnError(se) will eventually lead to close() being called
on the result set. Since the XPLAIN tables are updated on close(), we'll get an entry for
the result set even if it was never actually executed. Since GPS will re-execute the statement,
we'll get a second entry for the actual execution.

I think this will only happen if the statement is invalidated between GPS's call to rePrepare()
at line 409 and its call to resultSet.open() at line 436.

> 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
>            Priority: Minor
>         Attachments: 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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