Github user rxin commented on a diff in the pull request:
https://github.com/apache/spark/pull/7687#discussion_r35510553
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/expressions/udaf.scala ---
@@ -0,0 +1,97 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.expressions
+
+import org.apache.spark.sql.Row
+import org.apache.spark.sql.types._
+import org.apache.spark.annotation.Experimental
+
+/**
+ * :: Experimental ::
+ * The abstract class for implementing user-defined aggregate function.
+ */
+@Experimental
+abstract class UserDefinedAggregateFunction extends Serializable {
+
+ /**
+ * A [[StructType]] represents data types of input arguments of this
aggregate function.
+ * For example, if a [[UserDefinedAggregateFunction]] expects two input
arguments
+ * with type of [[DoubleType]] and [[LongType]], the returned
[[StructType]] will look like
+ *
+ * ```
+ * StructType(Seq(StructField("doubleInput", DoubleType),
StructField("longInput", LongType)))
+ * ```
+ *
+ * The name of a field of this [[StructType]] is only used to identify
the corresponding
+ * input argument. Users can choose names to identify the input
arguments.
+ */
+ def inputSchema: StructType
+
+ /**
+ * A [[StructType]] represents data types of values in the aggregation
buffer.
+ * For example, if a [[UserDefinedAggregateFunction]]'s buffer has two
values
+ * (i.e. two intermediate values) with type of [[DoubleType]] and
[[LongType]],
+ * the returned [[StructType]] will look like
+ *
+ * ```
+ * StructType(Seq(StructField("doubleInput", DoubleType),
StructField("longInput", LongType)))
--- End diff --
here too
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