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https://issues.apache.org/jira/browse/CALCITE-2798?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16750346#comment-16750346
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Julian Hyde commented on CALCITE-2798:
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The {{union}} query isn't a clear win. Eliminating duplicates is easier if the
input is sorted, therefore it is not a total waste of effort. Removing the sort
is a cost-based decision.
SQL semantics can safely ignore ORDER BY unless is at the top-level, so I
suppose the sql-to-rel converter could just drop the ORDER BY in queries 2 and
3. There is no guarantee that those queries return sorted results.
> Optimizer should remove unneeded Sort
> -------------------------------------
>
> 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
>
> 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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