[jira] [Commented] (SPARK-27290) remove unneed sort under Aggregate

["Xiaoju Wu (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-27290?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16862698#comment-16862698 ] 

Xiaoju Wu commented on SPARK-27290:
-----------------------------------

[~joshrosen] Got it. I think we should identify in which patterns sort is really needed and fix the UT to be more meaningful.

> 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: 3.0.0
>            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:
> {code}
> select count(*) from (select a1 from A order by a2);
> +- Aggregate
>   +- Sort
>      +- FileScan parquet
> {code}
> But one of the existing test cases is conflict with this example:
> {code}
> 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) 
> }
> {code}
> 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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