b-slim commented on issue #6814: [Discuss] Replacing hyperUnique as 'default' 
distinct count sketch
URL: 
https://github.com/apache/incubator-druid/issues/6814#issuecomment-452056269
 
 
   > Druid SQL's COUNT(DISTINCT x) operator uses hyperUnique currently.
   
   This is side comment, but i think it is miss leading to expose the 
approximate count distinct as the default count distinct since it does not 
adhere to SQL standard.
   
   Now to the issue of moving form Hyper unique to HllSketch I am kind of sure 
this kind of question will re occur again and again and every-time that a new 
approximate method outperform a an old one or maybe offers different tradeoffs. 
This tells me that probably the best way to solve this is to add a built in UDF 
for every different sketch algorithm with its respective parameter, this will 
give the user access to all the core supported sketches without issue of 
compatibilities.   
   
   > The fact that the new one is in an extension and the old one is in core 
presents the opportunity for user confusion. Ideally they'd both be in core or 
both be in extensions.
   
   Having HllSketch as part of core makes perfect sense to me the library has 
minimal dependency and does very well on what is suppose to do

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