[SPARK-7440][SQL] Remove physical Distinct operator in favor of Aggregate

This patch replaces Distinct with Aggregate in the optimizer, so Distinct will become
more efficient over time as we optimize Aggregate (via Tungsten).

Author: Reynold Xin <rxin@databricks.com>

Closes #6637 from rxin/replace-distinct and squashes the following commits:

b3cc50e [Reynold Xin] Mima excludes.
93d6117 [Reynold Xin] Code review feedback.
87e4741 [Reynold Xin] [SPARK-7440][SQL] Remove physical Distinct operator in favor of Aggregate.

project/MimaExcludes.scala

@@ -46,7 +46,9 @@ object MimaExcludes {
               "org.apache.spark.api.java.JavaRDDLike.partitioner"),
             // Mima false positive (was a private[spark] class)
             ProblemFilters.exclude[MissingClassProblem](
-              "org.apache.spark.util.collection.PairIterator")
+              "org.apache.spark.util.collection.PairIterator"),
+            // SQL execution is considered private.
+            excludePackage("org.apache.spark.sql.execution")
           )
         case v if v.startsWith("1.4") =>
           Seq(

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -36,6 +36,8 @@ object DefaultOptimizer extends Optimizer {
     // SubQueries are only needed for analysis and can be removed before execution.
     Batch("Remove SubQueries", FixedPoint(100),
       EliminateSubQueries) ::
+    Batch("Distinct", FixedPoint(100),
+      ReplaceDistinctWithAggregate) ::
     Batch("Operator Reordering", FixedPoint(100),
       UnionPushdown,
       CombineFilters,
@@ -696,3 +698,15 @@ object ConvertToLocalRelation extends Rule[LogicalPlan] {
       LocalRelation(projectList.map(_.toAttribute), data.map(projection))
   }
 }
+
+/**
+ * Replaces logical [[Distinct]] operator with an [[Aggregate]] operator.
+ * {{{
+ *   SELECT DISTINCT f1, f2 FROM t  ==>  SELECT f1, f2 FROM t GROUP BY f1, f2
+ * }}}
+ */
+object ReplaceDistinctWithAggregate extends Rule[LogicalPlan] {
+  def apply(plan: LogicalPlan): LogicalPlan = plan transform {
+    case Distinct(child) => Aggregate(child.output, child.output, child)
+  }
+}

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicOperators.scala

@@ -339,6 +339,9 @@ case class Sample(
   override def output: Seq[Attribute] = child.output
 }
 
+/**
+ * Returns a new logical plan that dedups input rows.
+ */
 case class Distinct(child: LogicalPlan) extends UnaryNode {
   override def output: Seq[Attribute] = child.output
 }

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/ReplaceDistinctWithAggregateSuite.scala

@@ -0,0 +1,42 @@
+/*
+ * 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.optimizer
+
+import org.apache.spark.sql.catalyst.dsl.plans._
+import org.apache.spark.sql.catalyst.dsl.expressions._
+import org.apache.spark.sql.catalyst.plans.PlanTest
+import org.apache.spark.sql.catalyst.plans.logical.{Aggregate, Distinct, LocalRelation, LogicalPlan}
+import org.apache.spark.sql.catalyst.rules.RuleExecutor
+
+class ReplaceDistinctWithAggregateSuite extends PlanTest {
+
+  object Optimize extends RuleExecutor[LogicalPlan] {
+    val batches = Batch("ProjectCollapsing", Once, ReplaceDistinctWithAggregate) :: Nil
+  }
+
+  test("replace distinct with aggregate") {
+    val input = LocalRelation('a.int, 'b.int)
+
+    val query = Distinct(input)
+    val optimized = Optimize.execute(query.analyze)
+
+    val correctAnswer = Aggregate(input.output, input.output, input)
+
+    comparePlans(optimized, correctAnswer)
+  }
+}

sql/core/src/main/scala/org/apache/spark/sql/DataFrame.scala

@@ -1311,7 +1311,7 @@ class DataFrame private[sql](
    * @group dfops
    * @since 1.3.0
    */
-  override def distinct: DataFrame = Distinct(logicalPlan)
+  override def distinct: DataFrame = dropDuplicates()
 
   /**
    * @group basic

sql/core/src/main/scala/org/apache/spark/sql/execution/SparkStrategies.scala

@@ -284,8 +284,8 @@ private[sql] abstract class SparkStrategies extends QueryPlanner[SparkPlan] {
       case r: RunnableCommand => ExecutedCommand(r) :: Nil
 
       case logical.Distinct(child) =>
-        execution.Distinct(partial = false,
-          execution.Distinct(partial = true, planLater(child))) :: Nil
+        throw new IllegalStateException(
+          "logical distinct operator should have been replaced by aggregate in the optimizer")
       case logical.Repartition(numPartitions, shuffle, child) =>
         execution.Repartition(numPartitions, shuffle, planLater(child)) :: Nil
       case logical.SortPartitions(sortExprs, child) =>

sql/core/src/main/scala/org/apache/spark/sql/execution/basicOperators.scala

@@ -230,37 +230,6 @@ case class ExternalSort(
   override def outputOrdering: Seq[SortOrder] = sortOrder
 }
 
-/**
- * :: DeveloperApi ::
- * Computes the set of distinct input rows using a HashSet.
- * @param partial when true the distinct operation is performed partially, per partition, without
- *                shuffling the data.
- * @param child the input query plan.
- */
-@DeveloperApi
-case class Distinct(partial: Boolean, child: SparkPlan) extends UnaryNode {
-  override def output: Seq[Attribute] = child.output
-
-  override def requiredChildDistribution: Seq[Distribution] =
-    if (partial) UnspecifiedDistribution :: Nil else ClusteredDistribution(child.output) :: Nil
-
-  protected override def doExecute(): RDD[Row] = {
-    child.execute().mapPartitions { iter =>
-      val hashSet = new scala.collection.mutable.HashSet[Row]()
-
-      var currentRow: Row = null
-      while (iter.hasNext) {
-        currentRow = iter.next()
-        if (!hashSet.contains(currentRow)) {
-          hashSet.add(currentRow.copy())
-        }
-      }
-
-      hashSet.iterator
-    }
-  }
-}
-
 /**
  * :: DeveloperApi ::
  * Return a new RDD that has exactly `numPartitions` partitions.