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https://issues.apache.org/jira/browse/HIVE-17976?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16243533#comment-16243533
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Rui Li commented on HIVE-17976:
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Seems the logic should only be implemented for map work, not reduce work. For
example, if we do a count() and there's no data in the table, we should return
0 instead of nothing. Therefore we should set output collector for the reduce
work even if it has no data to process. MR doesn't need to do this because
reducers of MR always end with operators that write to disk, e.g. FS.
> HoS: don't set output collector if there's no data to process
> -------------------------------------------------------------
>
> Key: HIVE-17976
> URL: https://issues.apache.org/jira/browse/HIVE-17976
> Project: Hive
> Issue Type: Bug
> Components: Spark
> Reporter: Rui Li
> Assignee: Rui Li
> Priority: Minor
> Attachments: HIVE-17976.1.patch, HIVE-17976.2.patch
>
>
> MR doesn't set an output collector if no row is processed, i.e.
> {{ExecMapper::map}} is never called. Let's investigate whether Spark should
> do the same.
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