Github user jaceklaskowski commented on a diff in the pull request:
https://github.com/apache/spark/pull/14576#discussion_r74706380
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/expressions/ReduceAggregator.scala
---
@@ -0,0 +1,79 @@
+/*
+ * 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.annotation.Experimental
+import org.apache.spark.sql.Encoder
+import org.apache.spark.sql.catalyst.encoders.ExpressionEncoder
+
+/**
+ * :: Experimental ::
+ * An aggregator that uses a single associative and commutative reduce
function. This reduce
+ * function can be used to go through all input values and reduces them to
a single value.
+ * If there is no input, a null value is returned.
+ *
+ * @since 2.1.0
+ */
+@Experimental
+abstract class ReduceAggregator[T] extends Aggregator[T, (Boolean, T), T] {
+
+ // Question 1: Should func and encoder be parameters rather than
abstract methods?
+ // rxin: abstract method has better java compatibility and forces
naming the concrete impl,
+ // whereas parameter has better type inference (infer encoders via
context bounds).
+ // Question 2: Should finish throw an exception or return null if there
is no input?
+ // rxin: null might be more "SQL" like, whereas exception is more Scala
like.
+
+ /**
+ * A associative and commutative reduce function.
+ * @since 2.1.0
+ */
+ def func(a: T, b: T): T
+
+ /**
+ * Encoder for type T.
+ * @since 2.1.0
+ */
+ def encoder: ExpressionEncoder[T]
+
+ override def zero: (Boolean, T) = (false, null.asInstanceOf[T])
+
+ override def bufferEncoder: Encoder[(Boolean, T)] =
+ ExpressionEncoder.tuple(ExpressionEncoder[Boolean](), encoder)
--- End diff --
`Encoders.scalaBoolean`?
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