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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