capistrant commented on issue #10042:
URL: https://github.com/apache/druid/issues/10042#issuecomment-675682218


   @gianm It appears to be essentially the same stack trace.
   
   @leerho since the sketch is only an aggregated value and not a persisted 
value in the segment, I only know how to access it as base64. If theres a tool 
out there to decode I will gladly do that, but I figured I'd talk with you 
before going down that route.
   
   recap: Druid 0.18 broker queries a druid 0.15 historical. So it would 
violate Lee's point above stating the only guarantee is that older sketches can 
be read by newer code and not the opposite as we have in this case.
   
   Base64 result from the historical node:
   ```JSON
   [
       {
           "timestamp": "2020-08-05T01:00:00.000Z",
           "result": {
               "historical_version_count_2week": "AQMDAAA6zJPXi9u+uHa0Uw==",
               "historical_version_count_1week": "AQMDAAA6zJPXi9u+uHa0Uw=="
           }
       }
   ]
   ```
   
   The query:
   ```JSON
   {
     "queryType": "timeseries",
     "dataSource": {
       "type": "table",
       "name": "druidMigration.cluster-metrics"
     },
     "granularity": "all",
     "intervals": [
         "2020-08-05T00:00:00/2020-08-19T00:00:00"
     ],
     "aggregations": [
       {
         "type": "filtered",
         "filter": {
           "type": "and",
           "fields": [
             {
               "type": "interval",
               "dimension": "__time",
               "intervals": [
                 "2020-08-05T00:00:00/2020-08-12T00:00:00"
               ]
             },
             {
               "type": "selector",
               "dimension": "service",
               "value": "druid/historical"
             }
           ]
         },
         "aggregator": {
           "type": "thetaSketch",
           "fieldName": "version",
           "name": "historical_version_count_2week"
         }
       },
       {
         "type": "filtered",
         "filter": {
           "type": "and",
           "fields": [
             {
               "type": "interval",
               "dimension": "__time",
               "intervals": [
                 "2020-08-12T00:00:00/2020-08-19T00:00:00"
               ]
             },
             {
               "type": "selector",
               "dimension": "service",
               "value": "druid/historical"
             }
           ]
         },
         "aggregator": {
           "type": "thetaSketch",
           "fieldName": "version",
           "name": "historical_version_count_1week"
         }
       }
     ],
     "postAggregations": [
       {
         "type": "arithmetic",
         "name": "(historical_version_count_2week - 
historical_version_count_1week)",
         "fn": "-",
         "fields": [
           {
             "type": "thetaSketchEstimate",
             "name": "historical_version_count_2week",
             "field": {
               "type": "fieldAccess",
               "fieldName": "historical_version_count_2week"
             }
           },
           {
             "type": "thetaSketchEstimate",
             "name": "historical_version_count_1week",
             "field": {
               "type": "fieldAccess",
               "fieldName": "historical_version_count_1week"
             }
           }
         ]
       }
     ],
     "filter": null,
     "context": {
       "priority": 2,
       "timeout": 60000,
       "useCache": false
     }
   }
   ```


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