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https://issues.apache.org/jira/browse/IMPALA-8845?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16905493#comment-16905493
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Sahil Takiar commented on IMPALA-8845:
--------------------------------------
CC: [~kwho]
Still trying to understand the code here, but looping in Michael as well.
> Close ExecNode tree prior to calling FlushFinal in FragmentInstanceState
> ------------------------------------------------------------------------
>
> Key: IMPALA-8845
> URL: https://issues.apache.org/jira/browse/IMPALA-8845
> Project: IMPALA
> Issue Type: Sub-task
> Components: Backend
> Reporter: Sahil Takiar
> Assignee: Sahil Takiar
> Priority: Major
>
> While testing IMPALA-8818, I found that IMPALA-8780 does not always cause all
> non-coordinator fragments to shutdown. In certain setups, TopN queries
> ({{select * from [table] order by [col] limit [limit]}}) where all results
> are successfully spooled, still keep non-coordinator fragments alive.
> The issue is that sometimes the {{DATASTREAM SINK}} for the TopN <-- Scan
> Node fragment ends up blocking waiting for a response to a {{TransmitData()}}
> RPC. This prevents the fragment from shutting down.
> I haven't traced the issue exactly, but what I *think* is happening is that
> the {{MERGING-EXCHANGE}} operator in the coordinator fragment hits {{eos}}
> whenever it has received enough rows to reach the limit defined in the query,
> which could occur before the {{DATASTREAM SINK}} sends all the rows from the
> TopN / Scan Node fragment.
> So the TopN / Scan Node fragments end up hanging until they are explicitly
> closed.
> The fix is to close the {{ExecNode}} tree in {{FragmentInstanceState}} as
> eagerly as possible. Moving the close call to before the call to
> {{DataSink::FlushFinal}} fixes the issue. It has the added benefit that it
> shuts down and releases all {{ExecNode}} resources as soon as it can. When
> result spooling is enabled, this is particularly important because
> {{FlushFinal}} might block until the consumer reads all rows.
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