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https://issues.apache.org/jira/browse/CALCITE-1670?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15895888#comment-15895888
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Julian Hyde commented on CALCITE-1670:
--------------------------------------
Yes, to support multiple distinct count you'd need support for GROUPING SETS
(or similar). Which wouldn't be a bad feature for Druid to have. :)
> Count distinct on druid is translated to Cardinality aggregator which is
> approximate
> ------------------------------------------------------------------------------------
>
> Key: CALCITE-1670
> URL: https://issues.apache.org/jira/browse/CALCITE-1670
> Project: Calcite
> Issue Type: Bug
> Reporter: Nishant Bangarwa
> Assignee: Julian Hyde
>
> Right now count distinct on Druid is translated as a 'cardinality' aggregator
> which uses hyperloglog and return approximate results. See cardinality
> aggregator here - http://druid.io/docs/latest/querying/aggregations.html for
> details.
> https://github.com/apache/calcite/blob/master/druid/src/main/java/org/apache/calcite/adapter/druid/DruidQuery.java#L721
> {code}
> case COUNT:
> if (aggCall.isDistinct()) {
> return new JsonCardinalityAggregation("cardinality", name, list);
> }
> return new JsonAggregation("count", name, only);
> {code}
> The current recommended way in druid to get exact counts is to do a nested
> groupby query.
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