[jira] [Created] (IMPALA-8845) Close ExecNode tree prior to calling FlushFinal in FragmentInstanceState

["Sahil Takiar (JIRA)" <ji...@apache.org>]

Sahil Takiar created IMPALA-8845:
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             Summary: 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


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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