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https://issues.apache.org/jira/browse/CALCITE-2798?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17101140#comment-17101140
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Julian Hyde commented on CALCITE-2798:
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I think the question is: if the {{ORDER BY}} is retained, will it affect the
results? For example,
{code}SELECT *
FROM emp
JOIN (SELECT * FROM dept ORDER BY deptno) USING (deptno){code}
is not guaranteed to produce results sorted by {{deptno}} and therefore the
optimizer should remove {{ORDER BY deptno}}. There are only two "relational
operators" I know where order matters - user-defined table functions and a user
viewing results of a query.
> Optimizer should remove ORDER BY in sub-query, provided it has no LIMIT or
> OFFSET
> ---------------------------------------------------------------------------------
>
> Key: CALCITE-2798
> URL: https://issues.apache.org/jira/browse/CALCITE-2798
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Affects Versions: 1.18.0
> Reporter: Vladimir Sitnikov
> Assignee: Julian Hyde
> Priority: Major
> Fix For: 1.19.0
>
>
> The following SQL performs sort twice, however inner sort can be eliminated
> {code}select * from (
> select * from "emps"
> order by "emps"."deptno"
> ) order by 1 desc{code}
> The same goes for (window calculation will sort on its own)
> {code}select row_number() over (order by "emps"."deptno") from (
> select * from "emps"
> order by "emps"."deptno" desc
> ){code}
> The same goes for SetOp (union, minus):
> {code}select * from (
> select * from "emps"
> order by "emps"."deptno"
> ) union select * from (
> select * from "emps"
> order by "emps"."deptno" desc
> ){code}
> There might be other cases like that (e.g. Aggregate, Join, Exchange,
> SortExchange)
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