Github user yhuai commented on a diff in the pull request: https://github.com/apache/spark/pull/15703#discussion_r88168751 --- Diff: sql/hive/src/main/scala/org/apache/spark/sql/hive/hiveUDFs.scala --- @@ -263,8 +265,19 @@ private[hive] case class HiveGenericUDTF( } /** - * Currently we don't support partial aggregation for queries using Hive UDAF, which may hurt - * performance a lot. + * While being evaluated by Spark SQL, the aggregation state of a Hive UDAF may be in the following + * three formats: + * + * 1. a Spark SQL value, or + * 2. an instance of some concrete `GenericUDAFEvaluator.AggregationBuffer` class, or + * 3. a Java object that can be inspected using the `ObjectInspector` returned by the + * `GenericUDAFEvaluator.init()` method. --- End diff -- Besides of explaining what are these three formats, let's also explain when we will use each of them.
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