[GitHub] [incubator-druid] Foxterran opened a new issue #8504: HLLSketchEstimateWithBounds can't be applied to Arithmetric post aggregator

[GitBox <gi...@apache.org>]

Foxterran opened a new issue #8504: HLLSketchEstimateWithBounds can't be applied to Arithmetric post aggregator
URL: https://github.com/apache/incubator-druid/issues/8504
 
 
   HLLSketchEstimateWithBounds can't be used as a numeric value in arithmetric post aggregator, because it's returning an array [estimate value, upper bounds, lower bounds]
   ### Affected Version
   
   0.14.2
   
   ### Description
   
   I have two HLLSketch metrics called unique_click_visitors and unique_view_visitors, it's built with "HLLSketchBuild" module during ingestion time, it's doing well via aggregator module, however, I need to calculate the ratio of the metrics, i.e. unique_click_visitors / unique_view_visitors to get the click through rate.
   
   Per this doc https://druid.apache.org/docs/latest/development/extensions-core/datasketches-hll.html , I started to use HLLSketchEstimateWithBounds  rounded with arithmetic post aggregator, the query spec (only postAggregations part) is as below:
   
   `"postAggregations": [
       {
         "type": "arithmetic",
         "name": "click_through_rate",
         "fn": "/",
         "fields": [
           {
             "type": "HLLSketchEstimateWithBounds",
             "name": "unique_click_visitors ",
             "field": {
               "type": "fieldAccess",
               "name": "unique_click_visitors ",
               "fieldName": "unique_click_visitors "
             }
           },
           {
             "type": "HLLSketchEstimateWithBounds",
             "name": "unique_view_visitors ",
             "field": {
               "type": "fieldAccess",
               "name": "unique_view_visitors ",
               "fieldName": "unique_view_visitors "
             }
           }
         ]
       }
     ]`
   
   However, it kept showing exception:
   `{
     "error": "Unknown exception",
     "errorMessage": "[D cannot be cast to java.lang.Number",
     "errorClass": "java.lang.ClassCastException",
     "host": null
   }`
   
   I looked at the source code,  https://github.com/apache/incubator-druid/blob/master/extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/hll/HllSketchToEstimateWithBoundsPostAggregator.java
   
   looks like this estimator is not only return the estimate value, but also its upper bound and lower bound, packaging the result as an array.
   
   I can bypass this error by doing the post aggregation in application layer, although.
   But anyone else suffering this issue? 
   Please help to share your way out here? 
   
   Thank YOU!

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