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https://issues.apache.org/jira/browse/CALCITE-2798?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17102082#comment-17102082
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Julian Hyde commented on CALCITE-2798:
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bq. other similar features in the standard, like top-level <order by clause> in
views (F852), confuse me even more.
Back when I used Oracle regularly, I found views with an ORDER BY clause to be
very convenient. I'd use them as "canned reports" for DBA stuff like
identifying tablespaces that were almost full.
ORDER BY sneaks into SQL as part of the "presentation layer", and that's how I
was using these views. I don't know what would have happened if I joined two of
these views, or applied SELECT DISTINCT or GROUP BY to them. But do I know that
they were convenient.
> 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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