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https://issues.apache.org/jira/browse/CALCITE-2798?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17102060#comment-17102060
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Stamatis Zampetakis commented on CALCITE-2798:
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To be honest, I am not sure. Nevertheless I cannot find a reason of why the
standard declares it as a feature if the optimizer is free to kill it at any
time. For the record the plan in Postgres for the query given by [~julianhyde]
is the following:
{noformat}
Merge Join (cost=246.68..480.99 rows=14351 width=44)
Merge Cond: (dept.deptno = emp.deptno)
-> Sort (cost=88.17..91.35 rows=1270 width=36)
Sort Key: dept.deptno
-> Seq Scan on dept (cost=0.00..22.70 rows=1270 width=36)
-> Sort (cost=158.51..164.16 rows=2260 width=8)
Sort Key: emp.deptno
-> Seq Scan on emp (cost=0.00..32.60 rows=2260 width=8)
{noformat}
On top of that there are other similar features in the standard, like top-level
<order by clause> in views (F852), that confuse me even more. By definition a
relation is unordered and a view is a relation so what does it mean to have an
order by in there.
Anyways if I find a clear answer I will come back again to this.
> 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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