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https://issues.apache.org/jira/browse/SPARK-12704?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15088638#comment-15088638
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Michael Armbrust commented on SPARK-12704:
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I think this explanation might be clearer with with actual queries and their
plans. That said "wrong" is a pretty strong term. Do you mean sub-optimal?
> we may repartition a relation even it's not needed
> --------------------------------------------------
>
> Key: SPARK-12704
> URL: https://issues.apache.org/jira/browse/SPARK-12704
> Project: Spark
> Issue Type: Bug
> Components: SQL
> Reporter: Wenchen Fan
>
> The implementation of {{HashPartitioning.compatibleWith}} has been wrong for
> a while. Think of the following case:
> if {{table_a}} is hash partitioned by int column `i`, and {{table_b}} is also
> partitioned by int column `i`, logically these 2 partitionings are
> compatible. However, {{HashPartitioning.compatibleWith}} will return false
> for this case as the {{AttributeReference}} of column `i` between these 2
> tables have different expr ids.
> With this wrong result of {{HashPartitioning.compatibleWith}}, we will go
> into [this
> branch|https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/Exchange.scala#L390]
> and may add unnecessary shuffle.
> This won't impact correctness if the join keys are exactly the same with hash
> partitioning keys, as there’s still an opportunity to not partition that
> child in that branch:
> https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/Exchange.scala#L428
> However, if the join keys are a super-set of hash partitioning keys, for
> example, {{table_a}} and {{table_b}} are both hash partitioned by column `i`,
> and we wanna join them using column `i, j`, logically we don't need shuffle
> but in fact the 2 tables start out as partitioned only by `i` and is
> redundantly repartitioned by `i, j`.
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