[GitHub] [incubator-druid] pdeva edited a comment on issue #7337: DataSketches HLL is not a replacement for Cardinality aggregator

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pdeva edited a comment on issue #7337: DataSketches HLL is not a replacement for Cardinality aggregator
URL: https://github.com/apache/incubator-druid/issues/7337#issuecomment-477879520
 
 
   @jon-wei  ok using `HLLSketchBuild` does work. though the documentation for this could definitely be improved.
   
   That said there are still a couple functionalities from `Cardinality` missing:
   
   1. `Cardinality` can take multiple fields. `HLLSketchBuild` can only take 1. 
   2. The `byRow` calculation is missing as pointed by @gianm 
   
   As mentioned in the [druid 0.13. docs for `Cardinality`](http://druid.io/docs/latest/querying/aggregations), I wonder how these 2 examples would translate to `HllSketchBuild`. 
   
   **Examples for Cardinality**
   
   1. Determine the number of distinct countries people are living in or have come from.
   
   ```json
   {
     "type": "cardinality",
     "name": "distinct_countries",
     "fields": [ "country_of_origin", "country_of_residence" ]
   }
   ```
   
   2. Determine the number of distinct people (i.e. combinations of first and last name).
   
   ```json
   {
     "type": "cardinality",
     "name": "distinct_people",
     "fields": [ "first_name", "last_name" ],
     "byRow" : true
   }
   ```
   
   3. Determine the number of distinct starting characters of last names
   
   ```json
   {
     "type": "cardinality",
     "name": "distinct_last_name_first_char",
     "fields": [
       {
        "type" : "extraction",
        "dimension" : "last_name",
        "outputName" :  "last_name_first_char",
        "extractionFn" : { "type" : "substring", "index" : 0, "length" : 1 }
       }
     ],
     "byRow" : true
   }
   ```
   
   <hr/>
   
   If you can provide the answers, I am happy to update the HLLSketch documentation with the equivalents for these.
   
   

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