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https://issues.apache.org/jira/browse/CALCITE-732?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14546322#comment-14546322
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Julian Hyde edited comment on CALCITE-732 at 5/15/15 10:57 PM:
---------------------------------------------------------------
The query{code}select "department_id" as d,
count(distinct "gender", "education_level") as c0,
count(distinct "gender") as c1,
count(distinct "education_level") as c2
from foodmart_clone."employee"
group by "department_id";{code} can be evaluated using an equivalent query that
uses grouping sets:{code}select d,
count(case i when 0 then 1 else null end) as c0,
count(case i when 1 then 1 else null end) as c1,
count(case i when 2 then 1 else null end) as c2
from (
select "department_id" as d,
grouping_id("gender", "education_level") as i,
"gender" as g,
"education_level" as e
from foodmart_clone."employee"
group by "department_id",
grouping sets ("gender", "education_level", ("gender", "education_level")))
group by d;{code}
was (Author: julianhyde):
The query{code}select "department_id" as d,
count(distinct "gender", "education_level") as c0,
count(distinct "education_level") as c1,
count(distinct "gender") as c2
from foodmart_clone."employee"
group by "department_id";{code} can be evaluated using an equivalent query that
uses grouping sets:{code}select d,
count(case i when 0 then 1 else null end) as c0,
count(case i when 1 then 1 else null end) as c1,
count(case i when 2 then 1 else null end) as c2
from (
select "department_id" as d,
grouping_id("gender", "education_level") as i,
"gender" as g,
"education_level" as e
from foodmart_clone."employee"
group by "department_id",
grouping sets ("gender", "education_level", ("gender", "education_level")))
group by d;{code}
> 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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