Julian Hyde created CALCITE-732:
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Summary: 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
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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