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https://issues.apache.org/jira/browse/CALCITE-732?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14563328#comment-14563328
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Julian Hyde commented on CALCITE-732:
-------------------------------------
Fix can be previewed at
https://github.com/julianhyde/incubator-calcite/tree/732-grouping-sets-distinct-count.
> Implement multiple distinct-COUNT using GROUPING SETS
> -----------------------------------------------------
>
> Key: CALCITE-732
> URL: https://issues.apache.org/jira/browse/CALCITE-732
> Project: Calcite
> Issue Type: Bug
> Reporter: Julian Hyde
> Assignee: Julian Hyde
> Fix For: next
>
>
> Currently if a query has COUNT(DISTINCT x) and COUNT(DISTINCT y) we compute
> the distinct counts separately and combine them using a join. The join isn't
> too expensive (because usually the GROUP BY has only a few keys) but we make
> multiple scans over the base table.
> I think we could translate multiple distinct-counts into a GROUPING SETS
> query (i.e. an Aggregate with more than one element in the groupSets field).
> If the underlying engine can evaluate that efficiently, then we have saved
> ourselves a join and several scans.
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