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https://issues.apache.org/jira/browse/HIVE-1544?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12899096#action_12899096
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Ning Zhang commented on HIVE-1544:
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The JoinDesc already has a flag noOuterJoin to keep track if there are outer
joins involved in the join operator. Based on that we should set a flag in the
ReduceSinkDesc to indicate whether NULL-keyed rows will be filtered out.
> Filtering out NULL-keyed rows in ReduceSinkOperator when no outer join
> involved
> -------------------------------------------------------------------------------
>
> Key: HIVE-1544
> URL: https://issues.apache.org/jira/browse/HIVE-1544
> Project: Hadoop Hive
> Issue Type: Improvement
> Reporter: Ning Zhang
>
> As discussed in HIVE-741, if a plan indicates that a non-outer join is the
> first operator in the reducer, the ReduceSinkOperator should filter out (not
> sending) rows with NULL as keys since they will not generate any results
> anyways. This should save both bandwidth and processing power.
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