Add MultiDistinctToCrossJoin optimizer rule for parallel distinct aggregates #20942
Description
Is your feature request related to a problem or challenge?
Queries like count(DISTINCT hits.SearchPhrase), count(DISTINCT hits.MobilePhone), count(DISTINCT hits.MobilePhoneModel) still don't run in parallel as they can not be optimized using the SingleDistinctToGroupBy optimization.
Describe the solution you'd like
Change it to issue the distinct queries in a separate stage, joining the results (either using join or union but join has a advantage on memory usage and cache locality as it runs them one by one).
Describe alternatives you've considered
No response
Additional context
No response