[CALCITE-1670] Count distinct on druid is translated to Cardinality aggregator which is approximate

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Assignee: Unassigned
• Reporter: Nishant Bangarwa
• Created: 03/Mar/17 14:25
• Updated: 22/Feb/19 01:21

Description

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

 
case COUNT:
      if (aggCall.isDistinct()) {
        return new JsonCardinalityAggregation("cardinality", name, list);
      }
      return new JsonAggregation("count", name, only);

The current recommended way in druid to get exact counts is to do a nested groupby query.

Issue Links

• is related to: CALCITE-1588 Add SQL syntax to allow approximate LIMIT and distinct-COUNT
• is related to: CALCITE-1587 Druid adapter: topN returns approximate results
• relates to: CALCITE-1787 thetaSketch Support for Druid Adapter