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https://issues.apache.org/jira/browse/SPARK-9258?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14660579#comment-14660579
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Reynold Xin commented on SPARK-9258:
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Marking as won't fix for now. We cannot replace semi join with normal join,
because the semantics are different. However, we can reuse the same join maps
for the hashsets.
> Remove all semi join physical operator
> --------------------------------------
>
> Key: SPARK-9258
> URL: https://issues.apache.org/jira/browse/SPARK-9258
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Reporter: Reynold Xin
>
> We have 4 semi join operators. In this case, they are not very very
> necessary. We can still use an equi-join operator to do the join, and just
> not include any values from the other join.
> We waste a little bit space due to building a hash map rather than a hash
> set, but at the end of the day unless we are going to spend a lot of time
> optimizing hash set, our Tungsten hash map will be a lot more efficient than
> the hash set anyway. This way, semi-join automatically benefits from all the
> work we do in Tungsten.
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