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https://issues.apache.org/jira/browse/CALCITE-5894?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17752308#comment-17752308
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JingDas commented on CALCITE-5894:
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As the above discussion, I think we have some consensus, I will add
FunctionalDependency in this Jira case, and give it the minimal implementation
in RelMdFunctionalDependency to satisfy the need of `SortRemoveRedundantRule`.
Then suppport more for functional dependency step by step in the future
togethor.
> Add SortRemoveRedundantRule to remove redundant sort fields if they are
> functionally dependent by other sort fields
> -------------------------------------------------------------------------------------------------------------------
>
> Key: CALCITE-5894
> URL: https://issues.apache.org/jira/browse/CALCITE-5894
> Project: Calcite
> Issue Type: New Feature
> Reporter: JingDas
> Assignee: JingDas
> Priority: Minor
> Attachments: image-2023-08-09-15-51-25-540.png
>
>
> In some scene, Sort fields can be reduct, if sort fields contain unique key
> For example
> {code:java}
> SELECT ename, salary FROM Emp
> order by empno, ename{code}
> where `empno` is a key, `ename` is redundant since `empno` alone is
> sufficient to determine the order of any two records.
> So the SQL can be optimized as following:
> {code:java}
> SELECT name, Emp.salary FROM Emp
> order by empno{code}
> For another example:
> {code:java}
> SELECT e_agg.c, e_agg.ename
> FROM
> (SELECT count(*) as c, ename, job FROM Emp GROUP BY ename, job) AS e_agg
> ORDER BY e_agg.ename, e_agg.c {code}
> Although `e_agg.ename` is not a key but field `ename` is unique and not null,
> it can be optimized as following:
> {code:java}
> SELECT e_agg.c, e_agg.ename
> FROM (SELECT count(*) as c, ename, job FROM Emp GROUP BY ename, job) AS e_agg
> ORDER BY e_agg.ename{code}
> Sorting is an expensive operation, however. Therefore, it is imperative that
> sorting
> is optimized to avoid unnecessary sort field.
>
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