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https://issues.apache.org/jira/browse/SPARK-27290?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16803737#comment-16803737
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Xiaoju Wu commented on SPARK-27290:
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[~ekoifman] HashAggregate can not benefit from sorted input but SortAggregate
can. But SortAggregate will require its input sorted by columns of group-by,
and will enforce the SortExec in the logic of EnsureRequirements. So the
user-written sort is useless.
> remove unneed sort under Aggregate
> ----------------------------------
>
> Key: SPARK-27290
> URL: https://issues.apache.org/jira/browse/SPARK-27290
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 2.4.0
> Reporter: Xiaoju Wu
> Priority: Minor
>
> I saw some tickets to remove unneeded sort in plan while I think there's
> another case in which sort is redundant:
> Sort just under an non-orderPreserving node is redundant, for example:
> select count(*) from (select a1 from A order by a2);
> +- Aggregate
> +- Sort
> +- FileScan parquet
> But one of the existing test cases is conflict with this example:
> test("sort should not be removed when there is a node which doesn't guarantee
> any order")
> { val orderedPlan = testRelation.select('a, 'b).orderBy('a.asc) val
> groupedAndResorted = orderedPlan.groupBy('a)(sum('a)).orderBy('a.asc) val
> optimized = Optimize.execute(groupedAndResorted.analyze) val correctAnswer
> = groupedAndResorted.analyze comparePlans(optimized, correctAnswer) }
> Why is it designed like this? In my opinion, since Aggregate won't pass up
> the ordering, the below Sort is useless.
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