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https://issues.apache.org/jira/browse/CALCITE-4021?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17114999#comment-17114999
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Julian Hyde commented on CALCITE-4021:
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{{EnumerableUnion}} COULD preserve ordering but SHOULD it? In order to preserve
ordering {{EnumerableUnion}} would have to merge. That would imply evaluating
the inputs in parallel and/or buffering, which might be less desirable in some
circumstances than what {{EnumerableUnion}} does today - namely a 'cat'
operation - read input 1 to completion, then read input 2 to completion.
I think we would not want to get rid of what we have today. So, to achieve what
you want we would implement {{EnumerableMergeUnion}}, which exactly what
CALCITE-3221 proposes.
> EnumerableUnion should preserve ordering when all inputs have the same
> collation
> --------------------------------------------------------------------------------
>
> Key: CALCITE-4021
> URL: https://issues.apache.org/jira/browse/CALCITE-4021
> Project: Calcite
> Issue Type: Improvement
> Reporter: Rui Wang
> Assignee: Rui Wang
> Priority: Major
>
> For query:
> {code:sql}
> select name from sales_dept
> union
> select name from it_dept
> order by mgr
> {code}
> We could generate a plan based on
> https://issues.apache.org/jira/browse/CALCITE-4017:
> {code:java}
> EnumerableUnion
> EnumerableProject
> EnumerableSort
> EnumerableTableScan[name="sales_dept"]
> EnumerableProject
> EnumerableSort
> EnumerableTableScan[name="it_dept"]
> {code}
> So EnumerableUnion should preserve ordering when all input have the same
> collation to make sure result of query is correct.
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