jihoonson commented on issue #5566: Applying doubleSum over doubleFirst aggregations in nested groupBy fails URL: https://github.com/apache/incubator-druid/issues/5566#issuecomment-408567669 Hi all, `DoubleFirstAggregator` returns `SerializedPair`s which is the intermediate format to merging doubleFirst values from multiple historicals. So, to compute some aggregations like `sum` over `doubleFirst`, you first need to _finalize_ the intermediate result to be the final format which contains only the double value. You can use `expression` postAggregator to do this. Here is an example. ```json { "queryType": "groupBy", "dataSource": { "type": "query", "query": { "queryType": "groupBy", "dataSource": "twitter", "aggregations": [ { "type": "doubleFirst", "name": "tweets", "fieldName": "tweets" } ], "postAggregations": [ { "type" : "expression", "name": "tweets_first", "expression" : "tweets" } ], "dimensions": [ "lang" ], "intervals": "2018-07-27/2018-07-28", "granularity": "hour" } }, "intervals": "2018-07-27/2018-07-28", "granularity": "hour", "aggregations": [ { "type": "doubleSum", "name": "value_num", "fieldName": "tweets_first" } ], "dimensions": [] } ```
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