Github user marmbrus commented on a diff in the pull request:
https://github.com/apache/spark/pull/9190#discussion_r42776824
--- Diff: sql/core/src/main/scala/org/apache/spark/sql/GroupedDataset.scala
---
@@ -0,0 +1,68 @@
+/*
+ * 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
+
+import org.apache.spark.sql.catalyst.encoders.Encoder
+import org.apache.spark.sql.catalyst.expressions.Attribute
+import org.apache.spark.sql.catalyst.plans.logical._
+import org.apache.spark.sql.execution.QueryExecution
+
+/**
+ * A [[Dataset]] has been logically grouped by a user specified grouping
key. Users should not
+ * construct a [[GroupedDataset]] directly, but should instead call
`groupBy` on an existing
+ * [[Dataset]].
+ */
+class GroupedDataset[K, T] private[sql](
+ private val kEncoder: Encoder[K],
+ private val tEncoder: Encoder[T],
+ queryExecution: QueryExecution,
+ private val dataAttributes: Seq[Attribute],
+ private val groupingAttributes: Seq[Attribute]) extends Serializable {
+
+ private implicit def kEnc = kEncoder
+ private implicit def tEnc = tEncoder
+ private def logicalPlan = queryExecution.analyzed
+ private def sqlContext = queryExecution.sqlContext
+
+ /**
+ * Returns a [[Dataset]] that contains each unique key.
+ */
+ def keys: Dataset[K] = {
--- End diff --
We don't *need* it, but its virtually free to implement efficiently,
additionally:
- [Scalding has
it](https://github.com/twitter/scalding/wiki/Type-safe-api-reference#creating-groups-and-joining-cogrouping)
- [PairRDD has
it](https://spark.apache.org/docs/0.7.2/api/core/spark/PairRDDFunctions.html)
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