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https://issues.apache.org/jira/browse/SPARK-21998?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16172169#comment-16172169
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Maryann Xue commented on SPARK-21998:
-------------------------------------
Thanks again for your comment, [~maropu]! I changed the title and description
of this JIRA accordingly and created a PR as
https://github.com/apache/spark/pull/19281. Could you please take a look?
> SortMergeJoinExec did not calculate its outputOrdering correctly during
> physical planning
> -----------------------------------------------------------------------------------------
>
> Key: SPARK-21998
> URL: https://issues.apache.org/jira/browse/SPARK-21998
> Project: Spark
> Issue Type: Bug
> Components: SQL
> Affects Versions: 2.2.0
> Reporter: Maryann Xue
> Priority: Minor
>
> Right now the calculation of SortMergeJoinExec's outputOrdering relies on the
> fact that its children have already been sorted on the join keys, while this
> is often not true until EnsureRequirements has been applied.
> {code}
> /**
> * For SMJ, child's output must have been sorted on key or expressions with
> the same order as
> * key, so we can get ordering for key from child's output ordering.
> */
> private def getKeyOrdering(keys: Seq[Expression], childOutputOrdering:
> Seq[SortOrder])
> : Seq[SortOrder] = {
> keys.zip(childOutputOrdering).map { case (key, childOrder) =>
> SortOrder(key, Ascending, childOrder.sameOrderExpressions +
> childOrder.child - key)
> }
> }
> {code}
> Thus SortMergeJoinExec's outputOrdering is most likely not correct during the
> physical planning stage, and as a result, potential physical optimizations
> that rely on the required/output orderings, like SPARK-18591, will not work
> for SortMergeJoinExec.
> The right behavior of {{getKeyOrdering(keys, childOutputOrdering)}} should be:
> 1. If the childOutputOrdering satisfies (is a superset of) the required child
> ordering => childOutputOrdering
> 2. Otherwise => required child ordering
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