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https://issues.apache.org/jira/browse/SPARK-32649?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17179397#comment-17179397
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Leanken.Lin commented on SPARK-32649:
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Feel free to just send out PR for reviewing，^_^
> Optimize BHJ/SHJ inner and semi join with empty hashed relation
> ---------------------------------------------------------------
>
> Key: SPARK-32649
> URL: https://issues.apache.org/jira/browse/SPARK-32649
> Project: Spark
> Issue Type: Sub-task
> Components: SQL
> Affects Versions: 3.1.0
> Reporter: Cheng Su
> Priority: Trivial
>
> With `EmptyHashedRelation` introduced in
> [https://github.com/apache/spark/pull/29389], it inspired me that there's a
> minor optimization we can apply to broadcast hash join and shuffled hash join
> if build side hashed relation is empty.
> If build side hashed relation is empty (i.e. build side is empty)
> 1.inner join: we don't need to execute stream side at all, just return an
> empty iterator -
> [https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/joins/HashJoin.scala#L152]
> 2.semi join: we don't need to execute stream side at all, just return an
> empty iterator -
> [https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/joins/HashJoin.scala#L227]
> .
> This is not common that build side is empty, but in case it is, we can
> leverage it to not execute stream side at all for better query CPU/IO
> performance.
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