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https://issues.apache.org/jira/browse/CALCITE-5717?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17724756#comment-17724756
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Julian Hyde commented on CALCITE-5717:
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These rewrites have tricky semantics. First, "select 1 from emps group by ()"
returns different results from "select distinct 1 from emps", for reasons that
are very subtle. Second, we don't want to be introducing RelNodes that have
zero fields.
I have done some work on this in CALCITE-4334 and I think we should have the
discussion there. LITERAL_AGG function allows us to create an Aggregate that
projects a constant, and therefore avoid the problem of zero-width RelNodes.
> Prune Project's input if project has no InputRef and the input has 1 row
> ------------------------------------------------------------------------
>
> Key: CALCITE-5717
> URL: https://issues.apache.org/jira/browse/CALCITE-5717
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Reporter: Benchao Li
> Assignee: Benchao Li
> Priority: Major
> Labels: pull-request-available
>
> The idea is to improve SQL
> {code:sql}
> select 1 from emps group by ()
> {code}
> In this case, we can safely prune the Aggregate.
> This idea is brought up in the discussion of CALCITE-5506
> We can add this improvement to ProjectAggregateMergeRule
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