[SPARK-17106] [SQL] Simplify the SubqueryExpression interface

## What changes were proposed in this pull request?
The current subquery expression interface contains a little bit of technical debt in the form of a few different access paths to get and set the query contained by the expression. This is confusing to anyone who goes over this code.

This PR unifies these access paths.

## How was this patch tested?
(Existing tests)

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #14685 from hvanhovell/SPARK-17106.

8 files changed, 56 insertions, 74 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala
index bd4c19181f..f540816366 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala
@@ -146,7 +146,7 @@ class Analyzer(
           // This cannot be done in ResolveSubquery because ResolveSubquery does not know the CTE.
           other transformExpressions {
             case e: SubqueryExpression =>
-              e.withNewPlan(substituteCTE(e.query, cteRelations))
+              e.withNewPlan(substituteCTE(e.plan, cteRelations))
           }
       }
     }
@@ -1091,7 +1091,7 @@ class Analyzer(
         f: (LogicalPlan, Seq[Expression]) => SubqueryExpression): SubqueryExpression = {
       // Step 1: Resolve the outer expressions.
       var previous: LogicalPlan = null
-      var current = e.query
+      var current = e.plan
       do {
         // Try to resolve the subquery plan using the regular analyzer.
         previous = current
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/subquery.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/subquery.scala
index ddbe937cba..e2e7d98e33 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/subquery.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/subquery.scala
@@ -17,33 +17,33 @@
 
 package org.apache.spark.sql.catalyst.expressions
 
-import org.apache.spark.sql.catalyst.analysis.TypeCheckResult
 import org.apache.spark.sql.catalyst.plans.QueryPlan
-import org.apache.spark.sql.catalyst.plans.logical.{LogicalPlan, SubqueryAlias}
+import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 import org.apache.spark.sql.types._
 
 /**
- * An interface for subquery that is used in expressions.
+ * An interface for expressions that contain a [[QueryPlan]].
  */
-abstract class SubqueryExpression extends Expression {
+abstract class PlanExpression[T <: QueryPlan[_]] extends Expression {
   /**  The id of the subquery expression. */
   def exprId: ExprId
 
-  /** The logical plan of the query. */
-  def query: LogicalPlan
+  /** The plan being wrapped in the query. */
+  def plan: T
 
-  /**
-   * Either a logical plan or a physical plan. The generated tree string (explain output) uses this
-   * field to explain the subquery.
-   */
-  def plan: QueryPlan[_]
-
-  /** Updates the query with new logical plan. */
-  def withNewPlan(plan: LogicalPlan): SubqueryExpression
+  /** Updates the expression with a new plan. */
+  def withNewPlan(plan: T): PlanExpression[T]
 
   protected def conditionString: String = children.mkString("[", " && ", "]")
 }
 
+/**
+ * A base interface for expressions that contain a [[LogicalPlan]].
+ */
+abstract class SubqueryExpression extends PlanExpression[LogicalPlan] {
+  override def withNewPlan(plan: LogicalPlan): SubqueryExpression
+}
+
 object SubqueryExpression {
   def hasCorrelatedSubquery(e: Expression): Boolean = {
     e.find {
@@ -60,20 +60,19 @@ object SubqueryExpression {
  * Note: `exprId` is used to have a unique name in explain string output.
  */
 case class ScalarSubquery(
-    query: LogicalPlan,
+    plan: LogicalPlan,
     children: Seq[Expression] = Seq.empty,
     exprId: ExprId = NamedExpression.newExprId)
   extends SubqueryExpression with Unevaluable {
-  override lazy val resolved: Boolean = childrenResolved && query.resolved
+  override lazy val resolved: Boolean = childrenResolved && plan.resolved
   override lazy val references: AttributeSet = {
-    if (query.resolved) super.references -- query.outputSet
+    if (plan.resolved) super.references -- plan.outputSet
     else super.references
   }
-  override def dataType: DataType = query.schema.fields.head.dataType
+  override def dataType: DataType = plan.schema.fields.head.dataType
   override def foldable: Boolean = false
   override def nullable: Boolean = true
-  override def plan: LogicalPlan = SubqueryAlias(toString, query, None)
-  override def withNewPlan(plan: LogicalPlan): ScalarSubquery = copy(query = plan)
+  override def withNewPlan(plan: LogicalPlan): ScalarSubquery = copy(plan = plan)
   override def toString: String = s"scalar-subquery#${exprId.id} $conditionString"
 }
 
@@ -92,19 +91,18 @@ object ScalarSubquery {
  * be rewritten into a left semi/anti join during analysis.
  */
 case class PredicateSubquery(
-    query: LogicalPlan,
+    plan: LogicalPlan,
     children: Seq[Expression] = Seq.empty,
     nullAware: Boolean = false,
     exprId: ExprId = NamedExpression.newExprId)
   extends SubqueryExpression with Predicate with Unevaluable {
-  override lazy val resolved = childrenResolved && query.resolved
-  override lazy val references: AttributeSet = super.references -- query.outputSet
+  override lazy val resolved = childrenResolved && plan.resolved
+  override lazy val references: AttributeSet = super.references -- plan.outputSet
   override def nullable: Boolean = nullAware
-  override def plan: LogicalPlan = SubqueryAlias(toString, query, None)
-  override def withNewPlan(plan: LogicalPlan): PredicateSubquery = copy(query = plan)
+  override def withNewPlan(plan: LogicalPlan): PredicateSubquery = copy(plan = plan)
   override def semanticEquals(o: Expression): Boolean = o match {
     case p: PredicateSubquery =>
-      query.sameResult(p.query) && nullAware == p.nullAware &&
+      plan.sameResult(p.plan) && nullAware == p.nullAware &&
         children.length == p.children.length &&
         children.zip(p.children).forall(p => p._1.semanticEquals(p._2))
     case _ => false
@@ -146,14 +144,13 @@ object PredicateSubquery {
  *                    FROM    b)
  * }}}
  */
-case class ListQuery(query: LogicalPlan, exprId: ExprId = NamedExpression.newExprId)
+case class ListQuery(plan: LogicalPlan, exprId: ExprId = NamedExpression.newExprId)
   extends SubqueryExpression with Unevaluable {
   override lazy val resolved = false
   override def children: Seq[Expression] = Seq.empty
   override def dataType: DataType = ArrayType(NullType)
   override def nullable: Boolean = false
-  override def withNewPlan(plan: LogicalPlan): ListQuery = copy(query = plan)
-  override def plan: LogicalPlan = SubqueryAlias(toString, query, None)
+  override def withNewPlan(plan: LogicalPlan): ListQuery = copy(plan = plan)
   override def toString: String = s"list#${exprId.id}"
 }
 
@@ -168,12 +165,11 @@ case class ListQuery(query: LogicalPlan, exprId: ExprId = NamedExpression.newExp
  *                   WHERE   b.id = a.id)
  * }}}
  */
-case class Exists(query: LogicalPlan, exprId: ExprId = NamedExpression.newExprId)
+case class Exists(plan: LogicalPlan, exprId: ExprId = NamedExpression.newExprId)
     extends SubqueryExpression with Predicate with Unevaluable {
   override lazy val resolved = false
   override def children: Seq[Expression] = Seq.empty
   override def nullable: Boolean = false
-  override def withNewPlan(plan: LogicalPlan): Exists = copy(query = plan)
-  override def plan: LogicalPlan = SubqueryAlias(toString, query, None)
+  override def withNewPlan(plan: LogicalPlan): Exists = copy(plan = plan)
   override def toString: String = s"exists#${exprId.id}"
 }
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
index f97a78b411..aa15f4a823 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
@@ -127,7 +127,7 @@ abstract class Optimizer(sessionCatalog: SessionCatalog, conf: CatalystConf)
   object OptimizeSubqueries extends Rule[LogicalPlan] {
     def apply(plan: LogicalPlan): LogicalPlan = plan transformAllExpressions {
       case s: SubqueryExpression =>
-        s.withNewPlan(Optimizer.this.execute(s.query))
+        s.withNewPlan(Optimizer.this.execute(s.plan))
     }
   }
 }
@@ -1814,7 +1814,7 @@ object RewriteCorrelatedScalarSubquery extends Rule[LogicalPlan] {
     val newExpression = expression transform {
       case s: ScalarSubquery if s.children.nonEmpty =>
         subqueries += s
-        s.query.output.head
+        s.plan.output.head
     }
     newExpression.asInstanceOf[E]
   }
@@ -2029,7 +2029,7 @@ object RewriteCorrelatedScalarSubquery extends Rule[LogicalPlan] {
         // grouping expressions. As a result we need to replace all the scalar subqueries in the
         // grouping expressions by their result.
         val newGrouping = grouping.map { e =>
-          subqueries.find(_.semanticEquals(e)).map(_.query.output.head).getOrElse(e)
+          subqueries.find(_.semanticEquals(e)).map(_.plan.output.head).getOrElse(e)
         }
         Aggregate(newGrouping, newExpressions, constructLeftJoins(child, subqueries))
       } else {
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/QueryPlan.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/QueryPlan.scala
index becf6945a2..8ee31f42ad 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/QueryPlan.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/QueryPlan.scala
@@ -263,7 +263,9 @@ abstract class QueryPlan[PlanType <: QueryPlan[PlanType]] extends TreeNode[PlanT
    * All the subqueries of current plan.
    */
   def subqueries: Seq[PlanType] = {
-    expressions.flatMap(_.collect {case e: SubqueryExpression => e.plan.asInstanceOf[PlanType]})
+    expressions.flatMap(_.collect {
+      case e: PlanExpression[_] => e.plan.asInstanceOf[PlanType]
+    })
   }
 
   override protected def innerChildren: Seq[QueryPlan[_]] = subqueries
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/catalyst/SQLBuilder.scala b/sql/core/src/main/scala/org/apache/spark/sql/catalyst/SQLBuilder.scala
index ff8e0f2642..0f51aa58d6 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/catalyst/SQLBuilder.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/catalyst/SQLBuilder.scala
@@ -80,7 +80,7 @@ class SQLBuilder private (
     try {
       val replaced = finalPlan.transformAllExpressions {
         case s: SubqueryExpression =>
-          val query = new SQLBuilder(s.query, nextSubqueryId, nextGenAttrId, exprIdMap).toSQL
+          val query = new SQLBuilder(s.plan, nextSubqueryId, nextGenAttrId, exprIdMap).toSQL
           val sql = s match {
             case _: ListQuery => query
             case _: Exists => s"EXISTS($query)"
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/subquery.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/subquery.scala
index c730bee6ae..730ca27f82 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/subquery.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/subquery.scala
@@ -22,9 +22,8 @@ import scala.collection.mutable.ArrayBuffer
 
 import org.apache.spark.sql.SparkSession
 import org.apache.spark.sql.catalyst.{expressions, InternalRow}
-import org.apache.spark.sql.catalyst.expressions._
+import org.apache.spark.sql.catalyst.expressions.{Expression, ExprId, InSet, Literal, PlanExpression}
 import org.apache.spark.sql.catalyst.expressions.codegen.{CodegenContext, ExprCode}
-import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 import org.apache.spark.sql.catalyst.rules.Rule
 import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.types.{BooleanType, DataType, StructType}
@@ -32,18 +31,7 @@ import org.apache.spark.sql.types.{BooleanType, DataType, StructType}
 /**
  * The base class for subquery that is used in SparkPlan.
  */
-trait ExecSubqueryExpression extends SubqueryExpression {
-
-  val executedPlan: SubqueryExec
-  def withExecutedPlan(plan: SubqueryExec): ExecSubqueryExpression
-
-  // does not have logical plan
-  override def query: LogicalPlan = throw new UnsupportedOperationException
-  override def withNewPlan(plan: LogicalPlan): SubqueryExpression =
-    throw new UnsupportedOperationException
-
-  override def plan: SparkPlan = executedPlan
-
+abstract class ExecSubqueryExpression extends PlanExpression[SubqueryExec] {
   /**
    * Fill the expression with collected result from executed plan.
    */
@@ -56,30 +44,29 @@ trait ExecSubqueryExpression extends SubqueryExpression {
  * This is the physical copy of ScalarSubquery to be used inside SparkPlan.
  */
 case class ScalarSubquery(
-    executedPlan: SubqueryExec,
+    plan: SubqueryExec,
     exprId: ExprId)
   extends ExecSubqueryExpression {
 
-  override def dataType: DataType = executedPlan.schema.fields.head.dataType
+  override def dataType: DataType = plan.schema.fields.head.dataType
   override def children: Seq[Expression] = Nil
   override def nullable: Boolean = true
-  override def toString: String = executedPlan.simpleString
-
-  def withExecutedPlan(plan: SubqueryExec): ExecSubqueryExpression = copy(executedPlan = plan)
+  override def toString: String = plan.simpleString
+  override def withNewPlan(query: SubqueryExec): ScalarSubquery = copy(plan = query)
 
   override def semanticEquals(other: Expression): Boolean = other match {
-    case s: ScalarSubquery => executedPlan.sameResult(executedPlan)
+    case s: ScalarSubquery => plan.sameResult(s.plan)
     case _ => false
   }
 
   // the first column in first row from `query`.
-  @volatile private var result: Any = null
+  @volatile private var result: Any = _
   @volatile private var updated: Boolean = false
 
   def updateResult(): Unit = {
     val rows = plan.executeCollect()
     if (rows.length > 1) {
-      sys.error(s"more than one row returned by a subquery used as an expression:\n${plan}")
+      sys.error(s"more than one row returned by a subquery used as an expression:\n$plan")
     }
     if (rows.length == 1) {
       assert(rows(0).numFields == 1,
@@ -108,7 +95,7 @@ case class ScalarSubquery(
  */
 case class InSubquery(
     child: Expression,
-    executedPlan: SubqueryExec,
+    plan: SubqueryExec,
     exprId: ExprId,
     private var result: Array[Any] = null,
     private var updated: Boolean = false) extends ExecSubqueryExpression {
@@ -116,13 +103,11 @@ case class InSubquery(
   override def dataType: DataType = BooleanType
   override def children: Seq[Expression] = child :: Nil
   override def nullable: Boolean = child.nullable
-  override def toString: String = s"$child IN ${executedPlan.name}"
-
-  def withExecutedPlan(plan: SubqueryExec): ExecSubqueryExpression = copy(executedPlan = plan)
+  override def toString: String = s"$child IN ${plan.name}"
+  override def withNewPlan(plan: SubqueryExec): InSubquery = copy(plan = plan)
 
   override def semanticEquals(other: Expression): Boolean = other match {
-    case in: InSubquery => child.semanticEquals(in.child) &&
-      executedPlan.sameResult(in.executedPlan)
+    case in: InSubquery => child.semanticEquals(in.child) && plan.sameResult(in.plan)
     case _ => false
   }
 
@@ -159,8 +144,8 @@ case class PlanSubqueries(sparkSession: SparkSession) extends Rule[SparkPlan] {
         ScalarSubquery(
           SubqueryExec(s"subquery${subquery.exprId.id}", executedPlan),
           subquery.exprId)
-      case expressions.PredicateSubquery(plan, Seq(e: Expression), _, exprId) =>
-        val executedPlan = new QueryExecution(sparkSession, plan).executedPlan
+      case expressions.PredicateSubquery(query, Seq(e: Expression), _, exprId) =>
+        val executedPlan = new QueryExecution(sparkSession, query).executedPlan
         InSubquery(e, SubqueryExec(s"subquery${exprId.id}", executedPlan), exprId)
     }
   }
@@ -184,9 +169,9 @@ case class ReuseSubquery(conf: SQLConf) extends Rule[SparkPlan] {
         val sameSchema = subqueries.getOrElseUpdate(sub.plan.schema, ArrayBuffer[SubqueryExec]())
         val sameResult = sameSchema.find(_.sameResult(sub.plan))
         if (sameResult.isDefined) {
-          sub.withExecutedPlan(sameResult.get)
+          sub.withNewPlan(sameResult.get)
         } else {
-          sameSchema += sub.executedPlan
+          sameSchema += sub.plan
           sub
         }
     }
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/QueryTest.scala b/sql/core/src/test/scala/org/apache/spark/sql/QueryTest.scala
index 304881d4a4..cff9d22d08 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/QueryTest.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/QueryTest.scala
@@ -292,7 +292,7 @@ abstract class QueryTest extends PlanTest {
         p.expressions.foreach {
           _.foreach {
             case s: SubqueryExpression =>
-              s.query.foreach(collectData)
+              s.plan.foreach(collectData)
             case _ =>
           }
         }
@@ -334,7 +334,7 @@ abstract class QueryTest extends PlanTest {
       case p =>
         p.transformExpressions {
           case s: SubqueryExpression =>
-            s.withNewPlan(s.query.transformDown(renormalize))
+            s.withNewPlan(s.plan.transformDown(renormalize))
         }
     }
     val normalized2 = jsonBackPlan.transformDown(renormalize)
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/execution/benchmark/TPCDSQueryBenchmark.scala b/sql/core/src/test/scala/org/apache/spark/sql/execution/benchmark/TPCDSQueryBenchmark.scala
index 957a1d6426..3988d9750b 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/execution/benchmark/TPCDSQueryBenchmark.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/execution/benchmark/TPCDSQueryBenchmark.scala
@@ -26,7 +26,6 @@ import org.apache.spark.sql.catalyst.analysis.UnresolvedRelation
 import org.apache.spark.sql.catalyst.expressions.SubqueryExpression
 import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 import org.apache.spark.sql.catalyst.util._
-import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.util.Benchmark
 
 /**