[CALCITE-732] Implement multiple distinct-COUNT using GROUPING SETS

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 1.4.0-incubating
• Assignee: Julian Hyde
• Reporter: Julian Hyde
• Created: 15/May/15 20:13
• Updated: 01/Sep/15 03:02
• Resolved: 01/Jun/15 18:32

Description

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.