[jira] [Commented] (CALCITE-2798) Optimizer should remove unneeded Sort

["Julian Hyde (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CALCITE-2798?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16750346#comment-16750346 ] 

Julian Hyde commented on CALCITE-2798:
--------------------------------------

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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