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Two questions about HLL


I am newbee of Druid, and I would like to aggregate hyperUnique of daily users to get the distinct count in a period of days, around this -

  1.  I am surprised by I did not even find the druid page about all of column types supported by Druid, so far what I met are - String, HyperUnique, LongSum, time etc;
  2.  Is there such post aggregation function to aggregate hyperUnique further to count the distinct values on the top of that? I did not find that it seems there is only one HyperUniqueCadinality for the count number from HyperUnique which can be used to arithmetical calculation only.

Thanks advance for your clarification.