Github user marmbrus commented on a diff in the pull request:
https://github.com/apache/spark/pull/7458#discussion_r34929775
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate2/aggregates.scala
---
@@ -0,0 +1,283 @@
+/*
+ * 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.catalyst.expressions.aggregate2
+
+import org.apache.spark.sql.catalyst.dsl.expressions._
+import org.apache.spark.sql.catalyst.errors.TreeNodeException
+import org.apache.spark.sql.catalyst.expressions._
+import org.apache.spark.sql.catalyst.{CatalystTypeConverters, InternalRow}
+import org.apache.spark.sql.types._
+import org.apache.spark.sql.Row
+
+/** The mode of an [[AggregateFunction]]. */
+private[sql] sealed trait AggregateMode
+
+/**
+ * An [[AggregateFunction]] with [[Partial]] mode is used for partial
aggregation.
+ * This function updates the given aggregation buffer with the original
input of this
+ * function. When it has processed all input rows, the aggregation buffer
is returned.
+ */
+private[sql] case object Partial extends AggregateMode
+
+/**
+ * An [[AggregateFunction]] with [[PartialMerge]] mode is used to merge
aggregation buffers
+ * containing intermediate results for this function.
+ * This function updates the given aggregation buffer by merging multiple
aggregation buffers.
+ * When it has processed all input rows, the aggregation buffer is
returned.
+ */
+private[sql] case object PartialMerge extends AggregateMode
+
+/**
+ * An [[AggregateFunction]] with [[PartialMerge]] mode is used to merge
aggregation buffers
+ * containing intermediate results for this function and the generate
final result.
+ * This function updates the given aggregation buffer by merging multiple
aggregation buffers.
+ * When it has processed all input rows, the final result of this function
is returned.
+ */
+private[sql] case object Final extends AggregateMode
+
+/**
+ * An [[AggregateFunction2]] with [[Partial]] mode is used to evaluate
this function directly
+ * from original input rows without any partial aggregation.
+ * This function updates the given aggregation buffer with the original
input of this
+ * function. When it has processed all input rows, the final result of
this function is returned.
+ */
+private[sql] case object Complete extends AggregateMode
+
+private[sql] case object NoOp extends Expression {
--- End diff --
We should move this out and comment it.
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