Foxterran
commented
5 years ago looks like we can use finalizingFieldAccess to solve this issue, changing the postAggregations part into:
"postAggregations": [ {"type": "arithmetic", "name": "click_through_rate", "fn": "/", "fields":[ {"type" : "finalizingFieldAccess", "name" : "unique_click_visitors ", "fieldName" : "unique_click_visitors " }, {"type" : "finalizingFieldAccess", "name" : "unique_view_visitors ", "fieldName" : "unique_view_visitors "} ] } ]
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!