[SPARK-12503] [SQL] Pushing Limit Through Union All

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

@@ -49,6 +49,7 @@ abstract class Optimizer extends RuleExecutor[LogicalPlan] {
       PushPredicateThroughProject,
       PushPredicateThroughGenerate,
       PushPredicateThroughAggregate,
+      PushDownLimit,
       ColumnPruning,
       // Operator combine
       ProjectCollapsing,
@@ -79,6 +80,33 @@ abstract class Optimizer extends RuleExecutor[LogicalPlan] {
   */
 object DefaultOptimizer extends Optimizer
 
+/**
+ * Pushes down Limit for reducing the amount of returned data.
+ *
+ * 1. Adding Extra Limit beneath the operations, including Union All.
+ * 2. Project is pushed through Limit in the rule ColumnPruning
+ *
+ * Any operator that a Limit can be pushed passed should override the maxRows function.
+ */
+object PushDownLimit extends Rule[LogicalPlan] {
+
+  def apply(plan: LogicalPlan): LogicalPlan = plan transform {
+
+    // Adding extra Limit below UNION ALL iff both left and right childs are not Limit or
+    // do not have Limit descendants. This heuristic is valid assuming there does not exist
+    // any Limit push-down rule that is unable to infer the value of maxRows.
+    // Note, right now, Union means UNION ALL, which does not de-duplicate rows. So, it is
+    // safe to pushdown Limit through it. Once we add UNION DISTINCT, we will not be able to
+    // pushdown Limit.
+    case Limit(exp, Union(left, right))
+      if left.maxRows.isEmpty || right.maxRows.isEmpty =>
+      Limit(exp,
+        Union(
+          Limit(exp, left),
+          Limit(exp, right)))
+  }
+}
+
 /**
  * Pushes operations down into a Sample.
  */
@@ -97,8 +125,8 @@ object SamplePushDown extends Rule[LogicalPlan] {
  * Operations that are safe to pushdown are listed as follows.
  * Union:
  * Right now, Union means UNION ALL, which does not de-duplicate rows. So, it is
- * safe to pushdown Filters and Projections through it. Once we add UNION DISTINCT,
- * we will not be able to pushdown Projections.
+ * safe to pushdown Filters, Projections and Limits through it. Once we add UNION DISTINCT,
+ * we will not be able to pushdown Projections and Limits.
  *
  * Intersect:
  * It is not safe to pushdown Projections through it because we need to get the

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

@@ -90,6 +90,13 @@ abstract class LogicalPlan extends QueryPlan[LogicalPlan] with Logging {
     Statistics(sizeInBytes = children.map(_.statistics.sizeInBytes).product)
   }
 
+  /**
+   * Returns the limited number of rows to be returned.
+   *
+   * Any operator that a Limit can be pushed passed should override this function.
+   */
+  def maxRows: Option[Expression] = None
+
   /**
    * Returns true if this expression and all its children have been resolved to a specific schema
    * and false if it still contains any unresolved placeholders. Implementations of LogicalPlan

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

@@ -28,6 +28,8 @@ import scala.collection.mutable.ArrayBuffer
 case class Project(projectList: Seq[NamedExpression], child: LogicalPlan) extends UnaryNode {
   override def output: Seq[Attribute] = projectList.map(_.toAttribute)
 
+  override def maxRows: Option[Expression] = child.maxRows
+
   override lazy val resolved: Boolean = {
     val hasSpecialExpressions = projectList.exists ( _.collect {
         case agg: AggregateExpression => agg
@@ -109,6 +111,9 @@ private[sql] object SetOperation {
 
 case class Union(left: LogicalPlan, right: LogicalPlan) extends SetOperation(left, right) {
 
+  override def maxRows: Option[Expression] =
+    for (leftMax <- left.maxRows; rightMax <- right.maxRows) yield Add(leftMax, rightMax)
+
   override def statistics: Statistics = {
     val sizeInBytes = left.statistics.sizeInBytes + right.statistics.sizeInBytes
     Statistics(sizeInBytes = sizeInBytes)
@@ -451,6 +456,8 @@ case class Pivot(
 case class Limit(limitExpr: Expression, child: LogicalPlan) extends UnaryNode {
   override def output: Seq[Attribute] = child.output
 
+  override def maxRows: Option[Expression] = Option(limitExpr)
+
   override lazy val statistics: Statistics = {
     val limit = limitExpr.eval().asInstanceOf[Int]
     val sizeInBytes = (limit: Long) * output.map(a => a.dataType.defaultSize).sum

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

@@ -0,0 +1,67 @@
+/*
+ * 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.analysis.EliminateSubQueries
+import org.apache.spark.sql.catalyst.plans.PlanTest
+import org.apache.spark.sql.catalyst.plans.logical._
+import org.apache.spark.sql.catalyst.rules._
+import org.apache.spark.sql.catalyst.dsl.plans._
+import org.apache.spark.sql.catalyst.dsl.expressions._
+
+class PushdownLimitsSuite extends PlanTest {
+  object Optimize extends RuleExecutor[LogicalPlan] {
+    val batches =
+      Batch("Subqueries", Once,
+        EliminateSubQueries) ::
+      Batch("Push Down Limit", Once,
+        PushDownLimit,
+        CombineLimits,
+        ConstantFolding,
+        BooleanSimplification) :: Nil
+  }
+
+  val testRelation = LocalRelation('a.int, 'b.int, 'c.int)
+  val testRelation2 = LocalRelation('d.int, 'e.int, 'f.int)
+
+  test("Union: limit to each side") {
+    val unionQuery = Union(testRelation, testRelation2).limit(1)
+    val unionOptimized = Optimize.execute(unionQuery.analyze)
+    val unionCorrectAnswer =
+      Limit(1, Union(testRelation.limit(1), testRelation2.limit(1))).analyze
+    comparePlans(unionOptimized, unionCorrectAnswer)
+  }
+
+  test("Union: limit to each side with the new limit number") {
+    val testLimitUnion = Union(testRelation, testRelation2.limit(3))
+    val unionQuery = testLimitUnion.limit(1)
+    val unionOptimized = Optimize.execute(unionQuery.analyze)
+    val unionCorrectAnswer =
+      Limit(1, Union(testRelation.limit(1), testRelation2.limit(1))).analyze
+    comparePlans(unionOptimized, unionCorrectAnswer)
+  }
+
+  test("Union: no limit to both sides") {
+    val testLimitUnion = Union(testRelation.limit(2), testRelation2.select('d).limit(3))
+    val unionQuery = testLimitUnion.limit(2)
+    val unionOptimized = Optimize.execute(unionQuery.analyze)
+    val unionCorrectAnswer =
+      Limit(2, Union(testRelation.limit(2), testRelation2.select('d).limit(3))).analyze
+    comparePlans(unionOptimized, unionCorrectAnswer)
+  }
+}