[SPARK-34969][SPARK-34906][SQL] Followup for Refactor TreeNode's children handling methods into specialized traits

### What changes were proposed in this pull request?

This is a followup for https://github.com/apache/spark/pull/31932.
In this PR we:
- Introduce the `QuaternaryLike` trait for node types with 4 children.
- Specialize more node types
- Fix a number of style errors that were introduced in the original PR.

### Why are the changes needed?

### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

This is a refactoring, passes existing tests.

Closes #32065 from dbaliafroozeh/FollowupSPARK-34906.

Authored-by: Ali Afroozeh <ali.afroozeh@databricks.com>
Signed-off-by: herman <herman@databricks.com>

mllib/src/main/scala/org/apache/spark/ml/stat/Summarizer.scala

@@ -28,6 +28,7 @@ import org.apache.spark.sql.Column
 import org.apache.spark.sql.catalyst.InternalRow
 import org.apache.spark.sql.catalyst.expressions.{Expression, ImplicitCastInputTypes}
 import org.apache.spark.sql.catalyst.expressions.aggregate.{AggregateExpression, Complete, TypedImperativeAggregate}
+import org.apache.spark.sql.catalyst.trees.BinaryLike
 import org.apache.spark.sql.functions.lit
 import org.apache.spark.sql.types._
 
@@ -348,7 +349,9 @@ private[spark] object SummaryBuilderImpl extends Logging {
       weightExpr: Expression,
       mutableAggBufferOffset: Int,
       inputAggBufferOffset: Int)
-    extends TypedImperativeAggregate[SummarizerBuffer] with ImplicitCastInputTypes {
+    extends TypedImperativeAggregate[SummarizerBuffer]
+    with ImplicitCastInputTypes
+    with BinaryLike[Expression] {
 
     override def eval(state: SummarizerBuffer): Any = {
       val metrics = requestedMetrics.map {
@@ -368,7 +371,8 @@ private[spark] object SummaryBuilderImpl extends Logging {
 
     override def inputTypes: Seq[DataType] = vectorUDT :: DoubleType :: Nil
 
-    override def children: Seq[Expression] = featuresExpr :: weightExpr :: Nil
+    override def left: Expression = featuresExpr
+    override def right: Expression = weightExpr
 
     override def update(state: SummarizerBuffer, row: InternalRow): SummarizerBuffer = {
       val features = vectorUDT.deserialize(featuresExpr.eval(row))

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala

@@ -24,7 +24,7 @@ import org.apache.spark.sql.catalyst.analysis.{FunctionRegistry, TypeCheckResult
 import org.apache.spark.sql.catalyst.expressions.aggregate.DeclarativeAggregate
 import org.apache.spark.sql.catalyst.expressions.codegen._
 import org.apache.spark.sql.catalyst.expressions.codegen.Block._
-import org.apache.spark.sql.catalyst.trees.{BinaryLike, LeafLike, TernaryLike, TreeNode, UnaryLike}
+import org.apache.spark.sql.catalyst.trees.{BinaryLike, LeafLike, QuaternaryLike, TernaryLike, TreeNode, UnaryLike}
 import org.apache.spark.sql.catalyst.util.truncatedString
 import org.apache.spark.sql.errors.QueryExecutionErrors
 import org.apache.spark.sql.internal.SQLConf
@@ -786,7 +786,7 @@ abstract class TernaryExpression extends Expression with TernaryLike[Expression]
  * An expression with four inputs and one output. The output is by default evaluated to null
  * if any input is evaluated to null.
  */
-abstract class QuaternaryExpression extends Expression {
+abstract class QuaternaryExpression extends Expression with QuaternaryLike[Expression] {
 
   override def foldable: Boolean = children.forall(_.foldable)
 
@@ -797,14 +797,13 @@ abstract class QuaternaryExpression extends Expression {
    * If subclass of QuaternaryExpression override nullable, probably should also override this.
    */
   override def eval(input: InternalRow): Any = {
-    val exprs = children
+    val value1 = first.eval(input)
-    val value1 = exprs(0).eval(input)
     if (value1 != null) {
-      val value2 = exprs(1).eval(input)
+      val value2 = second.eval(input)
       if (value2 != null) {
-        val value3 = exprs(2).eval(input)
+        val value3 = third.eval(input)
         if (value3 != null) {
-          val value4 = exprs(3).eval(input)
+          val value4 = fourth.eval(input)
           if (value4 != null) {
             return nullSafeEval(value1, value2, value3, value4)
           }

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/PartitionTransforms.scala

@@ -34,7 +34,7 @@ import org.apache.spark.sql.types.{DataType, IntegerType}
  * }}}
  */
 abstract class PartitionTransformExpression extends Expression with Unevaluable
-    with UnaryLike[Expression] {
+  with UnaryLike[Expression] {
   override def nullable: Boolean = true
 }
 

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Average.scala

@@ -36,7 +36,7 @@ import org.apache.spark.sql.types._
   group = "agg_funcs",
   since = "1.0.0")
 case class Average(child: Expression) extends DeclarativeAggregate with ImplicitCastInputTypes
-    with UnaryLike[Expression] {
+  with UnaryLike[Expression] {
 
   override def prettyName: String = getTagValue(FunctionRegistry.FUNC_ALIAS).getOrElse("avg")
 

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/CountIf.scala

@@ -36,7 +36,7 @@ import org.apache.spark.sql.types.{AbstractDataType, BooleanType, DataType, Long
   group = "agg_funcs",
   since = "3.0.0")
 case class CountIf(predicate: Expression) extends UnevaluableAggregate with ImplicitCastInputTypes
-    with UnaryLike[Expression] {
+  with UnaryLike[Expression] {
 
   override def prettyName: String = "count_if"
 

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/CountMinSketchAgg.scala

@@ -21,6 +21,7 @@ import org.apache.spark.sql.catalyst.InternalRow
 import org.apache.spark.sql.catalyst.analysis.TypeCheckResult
 import org.apache.spark.sql.catalyst.analysis.TypeCheckResult.{TypeCheckFailure, TypeCheckSuccess}
 import org.apache.spark.sql.catalyst.expressions.{ExpectsInputTypes, Expression, ExpressionDescription, Literal}
+import org.apache.spark.sql.catalyst.trees.QuaternaryLike
 import org.apache.spark.sql.types._
 import org.apache.spark.unsafe.types.UTF8String
 import org.apache.spark.util.sketch.CountMinSketch
@@ -60,7 +61,9 @@ case class CountMinSketchAgg(
     seedExpression: Expression,
     override val mutableAggBufferOffset: Int,
     override val inputAggBufferOffset: Int)
-  extends TypedImperativeAggregate[CountMinSketch] with ExpectsInputTypes {
+  extends TypedImperativeAggregate[CountMinSketch]
+  with ExpectsInputTypes
+  with QuaternaryLike[Expression] {
 
   def this(
       child: Expression,
@@ -145,8 +148,10 @@ case class CountMinSketchAgg(
   override def defaultResult: Option[Literal] =
     Option(Literal.create(eval(createAggregationBuffer()), dataType))
 
-  override def children: Seq[Expression] =
-    Seq(child, epsExpression, confidenceExpression, seedExpression)
-
   override def prettyName: String = "count_min_sketch"
+
+  override def first: Expression = child
+  override def second: Expression = epsExpression
+  override def third: Expression = confidenceExpression
+  override def fourth: Expression = seedExpression
 }

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Covariance.scala

@@ -27,11 +27,9 @@ import org.apache.spark.sql.types._
  * Compute the covariance between two expressions.
  * When applied on empty data (i.e., count is zero), it returns NULL.
  */
-abstract class Covariance(x: Expression, y: Expression, nullOnDivideByZero: Boolean)
+abstract class Covariance(val left: Expression, val right: Expression, nullOnDivideByZero: Boolean)
   extends DeclarativeAggregate with ImplicitCastInputTypes with BinaryLike[Expression] {
 
-  override def left: Expression = x
-  override def right: Expression = y
   override def nullable: Boolean = true
   override def dataType: DataType = DoubleType
   override def inputTypes: Seq[AbstractDataType] = Seq(DoubleType, DoubleType)
@@ -72,14 +70,14 @@ abstract class Covariance(x: Expression, y: Expression, nullOnDivideByZero: Bool
 
   protected def updateExpressionsDef: Seq[Expression] = {
     val newN = n + 1.0
-    val dx = x - xAvg
+    val dx = left - xAvg
-    val dy = y - yAvg
+    val dy = right - yAvg
     val dyN = dy / newN
     val newXAvg = xAvg + dx / newN
     val newYAvg = yAvg + dyN
-    val newCk = ck + dx * (y - newYAvg)
+    val newCk = ck + dx * (right - newYAvg)
 
-    val isNull = x.isNull || y.isNull
+    val isNull = left.isNull || right.isNull
     Seq(
       If(isNull, n, newN),
       If(isNull, xAvg, newXAvg),

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Sum.scala

@@ -39,7 +39,7 @@ import org.apache.spark.sql.types._
   group = "agg_funcs",
   since = "1.0.0")
 case class Sum(child: Expression) extends DeclarativeAggregate with ImplicitCastInputTypes
-    with UnaryLike[Expression] {
+  with UnaryLike[Expression] {
 
   override def nullable: Boolean = true
 

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/bitwiseAggregates.scala

@@ -22,7 +22,7 @@ import org.apache.spark.sql.catalyst.trees.UnaryLike
 import org.apache.spark.sql.types.{AbstractDataType, DataType, IntegralType}
 
 abstract class BitAggregate extends DeclarativeAggregate with ExpectsInputTypes
-    with UnaryLike[Expression] {
+  with UnaryLike[Expression] {
 
   val child: Expression
 

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/collect.scala

@@ -35,7 +35,7 @@ import org.apache.spark.sql.types._
  * can cause GC paused and eventually OutOfMemory Errors.
  */
 abstract class Collect[T <: Growable[Any] with Iterable[Any]] extends TypedImperativeAggregate[T]
-    with UnaryLike[Expression] {
+  with UnaryLike[Expression] {
 
   val child: Expression
 

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/grouping.scala

@@ -175,7 +175,7 @@ object GroupingSets {
   group = "agg_funcs")
 // scalastyle:on line.size.limit line.contains.tab
 case class Grouping(child: Expression) extends Expression with Unevaluable
-    with UnaryLike[Expression] {
+  with UnaryLike[Expression] {
   @transient
   override lazy val references: AttributeSet =
     AttributeSet(VirtualColumn.groupingIdAttribute :: Nil)

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/higherOrderFunctions.scala

@@ -25,6 +25,7 @@ import scala.collection.mutable
 import org.apache.spark.sql.catalyst.InternalRow
 import org.apache.spark.sql.catalyst.analysis.{TypeCheckResult, TypeCoercion, UnresolvedException}
 import org.apache.spark.sql.catalyst.expressions.codegen._
+import org.apache.spark.sql.catalyst.trees.{BinaryLike, QuaternaryLike, TernaryLike}
 import org.apache.spark.sql.catalyst.util._
 import org.apache.spark.sql.errors.QueryExecutionErrors
 import org.apache.spark.sql.internal.SQLConf
@@ -119,8 +120,6 @@ trait HigherOrderFunction extends Expression with ExpectsInputTypes {
 
   override def nullable: Boolean = arguments.exists(_.nullable)
 
-  override def children: Seq[Expression] = arguments ++ functions
-
   /**
    * Arguments of the higher ordered function.
    */
@@ -182,7 +181,7 @@ trait HigherOrderFunction extends Expression with ExpectsInputTypes {
 /**
  * Trait for functions having as input one argument and one function.
  */
-trait SimpleHigherOrderFunction extends HigherOrderFunction  {
+trait SimpleHigherOrderFunction extends HigherOrderFunction with BinaryLike[Expression] {
 
   def argument: Expression
 
@@ -202,6 +201,9 @@ trait SimpleHigherOrderFunction extends HigherOrderFunction  {
 
   def functionForEval: Expression = functionsForEval.head
 
+  override def left: Expression = argument
+  override def right: Expression = function
+
   /**
    * Called by [[eval]]. If a subclass keeps the default nullability, it can override this method
    * in order to save null-check code.
@@ -694,7 +696,7 @@ case class ArrayAggregate(
     zero: Expression,
     merge: Expression,
     finish: Expression)
-  extends HigherOrderFunction with CodegenFallback {
+  extends HigherOrderFunction with CodegenFallback with QuaternaryLike[Expression] {
 
   def this(argument: Expression, zero: Expression, merge: Expression) = {
     this(argument, zero, merge, LambdaFunction.identity)
@@ -760,6 +762,11 @@ case class ArrayAggregate(
   }
 
   override def prettyName: String = "aggregate"
+
+  override def first: Expression = argument
+  override def second: Expression = zero
+  override def third: Expression = merge
+  override def fourth: Expression = finish
 }
 
 /**
@@ -884,7 +891,7 @@ case class TransformValues(
   since = "3.0.0",
   group = "lambda_funcs")
 case class MapZipWith(left: Expression, right: Expression, function: Expression)
-  extends HigherOrderFunction with CodegenFallback {
+  extends HigherOrderFunction with CodegenFallback with TernaryLike[Expression] {
 
   def functionForEval: Expression = functionsForEval.head
 
@@ -1045,6 +1052,10 @@ case class MapZipWith(left: Expression, right: Expression, function: Expression)
   }
 
   override def prettyName: String = "map_zip_with"
+
+  override def first: Expression = left
+  override def second: Expression = right
+  override def third: Expression = function
 }
 
 // scalastyle:off line.size.limit
@@ -1063,7 +1074,7 @@ case class MapZipWith(left: Expression, right: Expression, function: Expression)
   group = "lambda_funcs")
 // scalastyle:on line.size.limit
 case class ZipWith(left: Expression, right: Expression, function: Expression)
-  extends HigherOrderFunction with CodegenFallback {
+  extends HigherOrderFunction with CodegenFallback with TernaryLike[Expression] {
 
   def functionForEval: Expression = functionsForEval.head
 
@@ -1071,7 +1082,7 @@ case class ZipWith(left: Expression, right: Expression, function: Expression)
 
   override def argumentTypes: Seq[AbstractDataType] = ArrayType :: ArrayType :: Nil
 
-  override def functions: Seq[Expression] = List(function)
+  override def functions: Seq[Expression] = function :: Nil
 
   override def functionTypes: Seq[AbstractDataType] = AnyDataType :: Nil
 
@@ -1121,4 +1132,8 @@ case class ZipWith(left: Expression, right: Expression, function: Expression)
   }
 
   override def prettyName: String = "zip_with"
+
+  override def first: Expression = left
+  override def second: Expression = right
+  override def third: Expression = function
 }

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/mathExpressions.scala

@@ -1488,7 +1488,6 @@ case class WidthBucket(
     numBucket: Expression)
   extends QuaternaryExpression with ImplicitCastInputTypes with NullIntolerant {
 
-  override def children: Seq[Expression] = Seq(value, minValue, maxValue, numBucket)
   override def inputTypes: Seq[AbstractDataType] = Seq(DoubleType, DoubleType, DoubleType, LongType)
   override def dataType: DataType = LongType
   override def nullable: Boolean = true
@@ -1507,4 +1506,9 @@ case class WidthBucket(
       "org.apache.spark.sql.catalyst.expressions.WidthBucket" +
         s".computeBucketNumber($input, $min, $max, $numBucket)")
   }
+
+  override def first: Expression = value
+  override def second: Expression = minValue
+  override def third: Expression = maxValue
+  override def fourth: Expression = numBucket
 }

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/regexpExpressions.scala

@@ -562,7 +562,6 @@ case class RegExpReplace(subject: Expression, regexp: Expression, rep: Expressio
   override def dataType: DataType = StringType
   override def inputTypes: Seq[AbstractDataType] =
     Seq(StringType, StringType, StringType, IntegerType)
-  override def children: Seq[Expression] = subject :: regexp :: rep :: pos :: Nil
   override def prettyName: String = "regexp_replace"
 
   override protected def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
@@ -618,6 +617,11 @@ case class RegExpReplace(subject: Expression, regexp: Expression, rep: Expressio
     """
     })
   }
+
+  override def first: Expression = subject
+  override def second: Expression = regexp
+  override def third: Expression = rep
+  override def fourth: Expression = pos
 }
 
 object RegExpReplace {

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/stringExpressions.scala

@@ -593,8 +593,6 @@ case class Overlay(input: Expression, replace: Expression, pos: Expression, len:
   override def inputTypes: Seq[AbstractDataType] = Seq(TypeCollection(StringType, BinaryType),
     TypeCollection(StringType, BinaryType), IntegerType, IntegerType)
 
-  override def children: Seq[Expression] = input :: replace :: pos :: len :: Nil
-
   override def checkInputDataTypes(): TypeCheckResult = {
     val inputTypeCheck = super.checkInputDataTypes()
     if (inputTypeCheck.isSuccess) {
@@ -631,6 +629,11 @@ case class Overlay(input: Expression, replace: Expression, pos: Expression, len:
       "org.apache.spark.sql.catalyst.expressions.Overlay" +
         s".calculate($input, $replace, $pos, $len);")
   }
+
+  override def first: Expression = input
+  override def second: Expression = replace
+  override def third: Expression = pos
+  override def fourth: Expression = len
 }
 
 object StringTranslate {

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/windowExpressions.scala

@@ -742,7 +742,7 @@ case class NthValue(input: Expression, offset: Expression, ignoreNulls: Boolean)
   group = "window_funcs")
 // scalastyle:on line.size.limit line.contains.tab
 case class NTile(buckets: Expression) extends RowNumberLike with SizeBasedWindowFunction
-    with UnaryLike[Expression] {
+  with UnaryLike[Expression] {
 
   def this() = this(Literal(1))
 

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/v2Commands.scala

@@ -431,7 +431,7 @@ case class InsertAction(
 }
 
 case class Assignment(key: Expression, value: Expression) extends Expression
-    with Unevaluable with BinaryLike[Expression] {
+  with Unevaluable with BinaryLike[Expression] {
   override def nullable: Boolean = false
   override def dataType: DataType = throw new UnresolvedException("nullable")
   override def left: Expression = key

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala

@@ -850,3 +850,11 @@ trait TernaryLike[T <: TreeNode[T]] { self: TreeNode[T] =>
   def third: T
   @transient override final lazy val children: Seq[T] = first :: second :: third :: Nil
 }
+
+trait QuaternaryLike[T <: TreeNode[T]] { self: TreeNode[T] =>
+  def first: T
+  def second: T
+  def third: T
+  def fourth: T
+  @transient override final lazy val children: Seq[T] = first :: second :: third :: fourth :: Nil
+}

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CodeGenerationSuite.scala

@@ -560,10 +560,9 @@ class CodeGenerationSuite extends SparkFunSuite with ExpressionEvalHelper {
   }
 }
 
-case class HugeCodeIntExpression(value: Int) extends Expression {
+case class HugeCodeIntExpression(value: Int) extends LeafExpression {
   override def nullable: Boolean = true
   override def dataType: DataType = IntegerType
-  override def children: Seq[Expression] = Nil
   override def eval(input: InternalRow): Any = value
   override def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {
     // Assuming HugeMethodLimit to be 8000

sql/core/src/main/scala/org/apache/spark/sql/execution/command/DataWritingCommand.scala

@@ -22,7 +22,7 @@ import org.apache.hadoop.conf.Configuration
 import org.apache.spark.SparkContext
 import org.apache.spark.sql.{Row, SparkSession}
 import org.apache.spark.sql.catalyst.expressions.Attribute
-import org.apache.spark.sql.catalyst.plans.logical.{Command, LogicalPlan}
+import org.apache.spark.sql.catalyst.plans.logical.{LogicalPlan, UnaryCommand}
 import org.apache.spark.sql.execution.{SparkPlan, SQLExecution}
 import org.apache.spark.sql.execution.datasources.BasicWriteJobStatsTracker
 import org.apache.spark.sql.execution.metric.{SQLMetric, SQLMetrics}
@@ -31,7 +31,7 @@ import org.apache.spark.util.SerializableConfiguration
 /**
  * A special `Command` which writes data out and updates metrics.
  */
-trait DataWritingCommand extends Command {
+trait DataWritingCommand extends UnaryCommand {
   /**
    * The input query plan that produces the data to be written.
    * IMPORTANT: the input query plan MUST be analyzed, so that we can carry its output columns
@@ -39,7 +39,7 @@ trait DataWritingCommand extends Command {
    */
   def query: LogicalPlan
 
-  override final def children: Seq[LogicalPlan] = query :: Nil
+  override final def child: LogicalPlan = query
 
   // Output column names of the analyzed input query plan.
   def outputColumnNames: Seq[String]

sql/core/src/main/scala/org/apache/spark/sql/execution/command/commands.scala

@@ -25,7 +25,7 @@ import org.apache.spark.sql.{Row, SparkSession}
 import org.apache.spark.sql.catalyst.{CatalystTypeConverters, InternalRow}
 import org.apache.spark.sql.catalyst.expressions.{Attribute, AttributeReference}
 import org.apache.spark.sql.catalyst.plans.QueryPlan
-import org.apache.spark.sql.catalyst.plans.logical.{LeafCommand, LogicalPlan}
+import org.apache.spark.sql.catalyst.plans.logical.{Command, LogicalPlan}
 import org.apache.spark.sql.connector.ExternalCommandRunner
 import org.apache.spark.sql.execution.{ExplainMode, LeafExecNode, SparkPlan, UnaryExecNode}
 import org.apache.spark.sql.execution.metric.SQLMetric
@@ -37,7 +37,9 @@ import org.apache.spark.sql.util.CaseInsensitiveStringMap
  * A logical command that is executed for its side-effects.  `RunnableCommand`s are
  * wrapped in `ExecutedCommand` during execution.
  */
-trait RunnableCommand extends LeafCommand {
+trait RunnableCommand extends Command {
+
+  override def children: Seq[LogicalPlan] = Nil
 
   // The map used to record the metrics of running the command. This will be passed to
   // `ExecutedCommand` during query planning.

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/AddPartitionExec.scala

@@ -31,7 +31,7 @@ case class AddPartitionExec(
     table: SupportsPartitionManagement,
     partSpecs: Seq[ResolvedPartitionSpec],
     ignoreIfExists: Boolean,
-    refreshCache: () => Unit) extends V2CommandExec {
+    refreshCache: () => Unit) extends LeafV2CommandExec {
   import DataSourceV2Implicits._
 
   override def output: Seq[Attribute] = Seq.empty

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/AlterNamespaceSetPropertiesExec.scala

@@ -27,7 +27,7 @@ import org.apache.spark.sql.connector.catalog.{NamespaceChange, SupportsNamespac
 case class AlterNamespaceSetPropertiesExec(
     catalog: SupportsNamespaces,
     namespace: Seq[String],
-    props: Map[String, String]) extends V2CommandExec {
+    props: Map[String, String]) extends LeafV2CommandExec {
   override protected def run(): Seq[InternalRow] = {
     val changes = props.map{ case (k, v) =>
       NamespaceChange.setProperty(k, v)

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/AlterTableExec.scala

@@ -28,7 +28,7 @@ import org.apache.spark.sql.errors.QueryExecutionErrors
 case class AlterTableExec(
     catalog: TableCatalog,
     ident: Identifier,
-    changes: Seq[TableChange]) extends V2CommandExec {
+    changes: Seq[TableChange]) extends LeafV2CommandExec {
 
   override def output: Seq[Attribute] = Seq.empty
 

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/CacheTableExec.scala

@@ -29,7 +29,7 @@ import org.apache.spark.sql.connector.catalog.CatalogV2Implicits.MultipartIdenti
 import org.apache.spark.sql.execution.command.CreateViewCommand
 import org.apache.spark.storage.StorageLevel
 
-trait BaseCacheTableExec extends V2CommandExec {
+trait BaseCacheTableExec extends LeafV2CommandExec {
   def relationName: String
   def planToCache: LogicalPlan
   def dataFrameForCachedPlan: DataFrame
@@ -117,7 +117,7 @@ case class CacheTableAsSelectExec(
 
 case class UncacheTableExec(
     relation: LogicalPlan,
-    cascade: Boolean) extends V2CommandExec {
+    cascade: Boolean) extends LeafV2CommandExec {
   override def run(): Seq[InternalRow] = {
     val sparkSession = sqlContext.sparkSession
     sparkSession.sharedState.cacheManager.uncacheQuery(sparkSession, relation, cascade)

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/CreateNamespaceExec.scala

@@ -34,7 +34,7 @@ case class CreateNamespaceExec(
     namespace: Seq[String],
     ifNotExists: Boolean,
     private var properties: Map[String, String])
-    extends V2CommandExec {
+    extends LeafV2CommandExec {
   override protected def run(): Seq[InternalRow] = {
     import org.apache.spark.sql.connector.catalog.CatalogV2Implicits._
     import org.apache.spark.sql.connector.catalog.SupportsNamespaces._

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/CreateTableExec.scala

@@ -33,7 +33,7 @@ case class CreateTableExec(
     tableSchema: StructType,
     partitioning: Seq[Transform],
     tableProperties: Map[String, String],
-    ignoreIfExists: Boolean) extends V2CommandExec {
+    ignoreIfExists: Boolean) extends LeafV2CommandExec {
   import org.apache.spark.sql.connector.catalog.CatalogV2Implicits._
 
   override protected def run(): Seq[InternalRow] = {

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/DeleteFromTableExec.scala

@@ -25,7 +25,7 @@ import org.apache.spark.sql.sources.Filter
 case class DeleteFromTableExec(
     table: SupportsDelete,
     condition: Array[Filter],
-    refreshCache: () => Unit) extends V2CommandExec {
+    refreshCache: () => Unit) extends LeafV2CommandExec {
 
   override protected def run(): Seq[InternalRow] = {
     table.deleteWhere(condition)

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/DescribeColumnExec.scala

@@ -25,7 +25,7 @@ import org.apache.spark.sql.catalyst.expressions.Attribute
 case class DescribeColumnExec(
     override val output: Seq[Attribute],
     column: Attribute,
-    isExtended: Boolean) extends V2CommandExec {
+    isExtended: Boolean) extends LeafV2CommandExec {
 
   override protected def run(): Seq[InternalRow] = {
     val rows = new ArrayBuffer[InternalRow]()

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/DescribeNamespaceExec.scala

@@ -31,7 +31,7 @@ case class DescribeNamespaceExec(
     output: Seq[Attribute],
     catalog: SupportsNamespaces,
     namespace: Seq[String],
-    isExtended: Boolean) extends V2CommandExec {
+    isExtended: Boolean) extends LeafV2CommandExec {
   override protected def run(): Seq[InternalRow] = {
     val rows = new ArrayBuffer[InternalRow]()
     val ns = namespace.toArray

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/DescribeTableExec.scala

@@ -27,7 +27,7 @@ import org.apache.spark.sql.connector.catalog.{CatalogV2Util, SupportsMetadataCo
 case class DescribeTableExec(
     output: Seq[Attribute],
     table: Table,
-    isExtended: Boolean) extends V2CommandExec {
+    isExtended: Boolean) extends LeafV2CommandExec {
   override protected def run(): Seq[InternalRow] = {
     val rows = new ArrayBuffer[InternalRow]()
     addSchema(rows)

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/DropNamespaceExec.scala

@@ -30,7 +30,7 @@ case class DropNamespaceExec(
     namespace: Seq[String],
     ifExists: Boolean,
     cascade: Boolean)
-  extends V2CommandExec {
+  extends LeafV2CommandExec {
   override protected def run(): Seq[InternalRow] = {
     import org.apache.spark.sql.connector.catalog.CatalogV2Implicits._
 

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/DropPartitionExec.scala

@@ -30,7 +30,7 @@ case class DropPartitionExec(
     partSpecs: Seq[ResolvedPartitionSpec],
     ignoreIfNotExists: Boolean,
     purge: Boolean,
-    refreshCache: () => Unit) extends V2CommandExec {
+    refreshCache: () => Unit) extends LeafV2CommandExec {
   import DataSourceV2Implicits._
 
   override def output: Seq[Attribute] = Seq.empty

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/DropTableExec.scala

@@ -30,7 +30,7 @@ case class DropTableExec(
     ident: Identifier,
     ifExists: Boolean,
     purge: Boolean,
-    invalidateCache: () => Unit) extends V2CommandExec {
+    invalidateCache: () => Unit) extends LeafV2CommandExec {
 
   override def run(): Seq[InternalRow] = {
     if (catalog.tableExists(ident)) {

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/RefreshTableExec.scala

@@ -24,7 +24,7 @@ import org.apache.spark.sql.connector.catalog.{Identifier, TableCatalog}
 case class RefreshTableExec(
     catalog: TableCatalog,
     ident: Identifier,
-    refreshCache: () => Unit) extends V2CommandExec {
+    refreshCache: () => Unit) extends LeafV2CommandExec {
   override protected def run(): Seq[InternalRow] = {
     catalog.invalidateTable(ident)
 

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/RenamePartitionExec.scala

@@ -29,7 +29,7 @@ case class RenamePartitionExec(
     table: SupportsPartitionManagement,
     from: ResolvedPartitionSpec,
     to: ResolvedPartitionSpec,
-    refreshCache: () => Unit) extends V2CommandExec {
+    refreshCache: () => Unit) extends LeafV2CommandExec {
 
   override def output: Seq[Attribute] = Seq.empty
 

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/RenameTableExec.scala

@@ -33,7 +33,7 @@ case class RenameTableExec(
     newIdent: Identifier,
     invalidateCache: () => Option[StorageLevel],
     cacheTable: (SparkSession, LogicalPlan, Option[String], StorageLevel) => Unit)
-  extends V2CommandExec {
+  extends LeafV2CommandExec {
 
   override def output: Seq[Attribute] = Seq.empty
 

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/ReplaceTableExec.scala

@@ -35,7 +35,7 @@ case class ReplaceTableExec(
     partitioning: Seq[Transform],
     tableProperties: Map[String, String],
     orCreate: Boolean,
-    invalidateCache: (TableCatalog, Table, Identifier) => Unit) extends V2CommandExec {
+    invalidateCache: (TableCatalog, Table, Identifier) => Unit) extends LeafV2CommandExec {
 
   override protected def run(): Seq[InternalRow] = {
     if (catalog.tableExists(ident)) {
@@ -59,7 +59,7 @@ case class AtomicReplaceTableExec(
     partitioning: Seq[Transform],
     tableProperties: Map[String, String],
     orCreate: Boolean,
-    invalidateCache: (TableCatalog, Table, Identifier) => Unit) extends V2CommandExec {
+    invalidateCache: (TableCatalog, Table, Identifier) => Unit) extends LeafV2CommandExec {
 
   override protected def run(): Seq[InternalRow] = {
     if (catalog.tableExists(identifier)) {

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/SetCatalogAndNamespaceExec.scala

@@ -28,7 +28,7 @@ case class SetCatalogAndNamespaceExec(
     catalogManager: CatalogManager,
     catalogName: Option[String],
     namespace: Option[Seq[String]])
-    extends V2CommandExec {
+    extends LeafV2CommandExec {
   override protected def run(): Seq[InternalRow] = {
     // The catalog is updated first because CatalogManager resets the current namespace
     // when the current catalog is set.

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/ShowCurrentNamespaceExec.scala

@@ -28,7 +28,7 @@ import org.apache.spark.sql.connector.catalog.CatalogV2Implicits.NamespaceHelper
 case class ShowCurrentNamespaceExec(
     output: Seq[Attribute],
     catalogManager: CatalogManager)
-  extends V2CommandExec {
+  extends LeafV2CommandExec {
   override protected def run(): Seq[InternalRow] = {
     Seq(toCatalystRow(catalogManager.currentCatalog.name, catalogManager.currentNamespace.quoted))
   }

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/ShowTablePropertiesExec.scala

@@ -27,7 +27,7 @@ import org.apache.spark.sql.connector.catalog.{CatalogV2Util, Table}
 case class ShowTablePropertiesExec(
     output: Seq[Attribute],
     catalogTable: Table,
-    propertyKey: Option[String]) extends V2CommandExec {
+    propertyKey: Option[String]) extends LeafV2CommandExec {
 
   override protected def run(): Seq[InternalRow] = {
     import scala.collection.JavaConverters._

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/TruncatePartitionExec.scala

@@ -28,7 +28,7 @@ import org.apache.spark.sql.connector.catalog.{SupportsAtomicPartitionManagement
 case class TruncatePartitionExec(
     table: SupportsPartitionManagement,
     partSpec: ResolvedPartitionSpec,
-    refreshCache: () => Unit) extends V2CommandExec {
+    refreshCache: () => Unit) extends LeafV2CommandExec {
 
   override def output: Seq[Attribute] = Seq.empty
 

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/TruncateTableExec.scala

@@ -26,7 +26,7 @@ import org.apache.spark.sql.connector.catalog.TruncatableTable
  */
 case class TruncateTableExec(
     table: TruncatableTable,
-    refreshCache: () => Unit) extends V2CommandExec {
+    refreshCache: () => Unit) extends LeafV2CommandExec {
 
   override def output: Seq[Attribute] = Seq.empty
 

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/V1FallbackWriters.scala

@@ -55,9 +55,8 @@ case class OverwriteByExpressionExecV1(
     write: V1Write) extends V1FallbackWriters
 
 /** Some helper interfaces that use V2 write semantics through the V1 writer interface. */
-sealed trait V1FallbackWriters extends V2CommandExec with SupportsV1Write {
+sealed trait V1FallbackWriters extends LeafV2CommandExec with SupportsV1Write {
   override def output: Seq[Attribute] = Nil
-  override final def children: Seq[SparkPlan] = Nil
 
   def table: SupportsWrite
   def refreshCache: () => Unit

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/V2CommandExec.scala

@@ -21,6 +21,7 @@ import org.apache.spark.rdd.RDD
 import org.apache.spark.sql.catalyst.InternalRow
 import org.apache.spark.sql.catalyst.encoders.RowEncoder
 import org.apache.spark.sql.catalyst.expressions.{AttributeSet, GenericRowWithSchema}
+import org.apache.spark.sql.catalyst.trees.LeafLike
 import org.apache.spark.sql.execution.SparkPlan
 import org.apache.spark.sql.types.StructType
 
@@ -57,8 +58,6 @@ abstract class V2CommandExec extends SparkPlan {
     sqlContext.sparkContext.parallelize(result, 1)
   }
 
-  override def children: Seq[SparkPlan] = Nil
-
   override def producedAttributes: AttributeSet = outputSet
 
   protected def toCatalystRow(values: Any*): InternalRow = {
@@ -69,3 +68,5 @@ abstract class V2CommandExec extends SparkPlan {
     RowEncoder(StructType.fromAttributes(output)).resolveAndBind().createSerializer()
   }
 }
+
+trait LeafV2CommandExec extends V2CommandExec with LeafLike[SparkPlan]

sql/core/src/test/scala/org/apache/spark/sql/DataFrameFunctionsSuite.scala

@@ -24,7 +24,7 @@ import scala.util.Random
 
 import org.apache.spark.SparkException
 import org.apache.spark.sql.catalyst.InternalRow
-import org.apache.spark.sql.catalyst.expressions.Expression
+import org.apache.spark.sql.catalyst.expressions.{Expression, UnaryExpression}
 import org.apache.spark.sql.catalyst.expressions.codegen.CodegenFallback
 import org.apache.spark.sql.catalyst.plans.logical.OneRowRelation
 import org.apache.spark.sql.catalyst.util.DateTimeTestUtils.{withDefaultTimeZone, UTC}
@@ -3632,8 +3632,7 @@ class DataFrameFunctionsSuite extends QueryTest with SharedSparkSession {
 }
 
 object DataFrameFunctionsSuite {
-  case class CodegenFallbackExpr(child: Expression) extends Expression with CodegenFallback {
+  case class CodegenFallbackExpr(child: Expression) extends UnaryExpression with CodegenFallback {
-    override def children: Seq[Expression] = Seq(child)
     override def nullable: Boolean = child.nullable
     override def dataType: DataType = child.dataType
     override lazy val resolved = true

sql/core/src/test/scala/org/apache/spark/sql/GeneratorFunctionSuite.scala

@@ -21,6 +21,7 @@ import org.apache.spark.sql.catalyst.InternalRow
 import org.apache.spark.sql.catalyst.expressions.{Expression, Generator}
 import org.apache.spark.sql.catalyst.expressions.codegen.{CodegenContext, ExprCode}
 import org.apache.spark.sql.catalyst.expressions.codegen.Block._
+import org.apache.spark.sql.catalyst.trees.LeafLike
 import org.apache.spark.sql.functions._
 import org.apache.spark.sql.test.SharedSparkSession
 import org.apache.spark.sql.types.{IntegerType, StructType}
@@ -358,8 +359,7 @@ class GeneratorFunctionSuite extends QueryTest with SharedSparkSession {
   }
 }
 
-case class EmptyGenerator() extends Generator {
+case class EmptyGenerator() extends Generator with LeafLike[Expression] {
-  override def children: Seq[Expression] = Nil
   override def elementSchema: StructType = new StructType().add("id", IntegerType)
   override def eval(input: InternalRow): TraversableOnce[InternalRow] = Seq.empty
   override protected def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = {

sql/core/src/test/scala/org/apache/spark/sql/TypedImperativeAggregateSuite.scala

@@ -23,6 +23,7 @@ import org.apache.spark.sql.TypedImperativeAggregateSuite.TypedMax
 import org.apache.spark.sql.catalyst.InternalRow
 import org.apache.spark.sql.catalyst.expressions.{BoundReference, Expression, GenericInternalRow, ImplicitCastInputTypes, SpecificInternalRow}
 import org.apache.spark.sql.catalyst.expressions.aggregate.TypedImperativeAggregate
+import org.apache.spark.sql.catalyst.trees.UnaryLike
 import org.apache.spark.sql.execution.aggregate.HashAggregateExec
 import org.apache.spark.sql.expressions.Window
 import org.apache.spark.sql.functions._
@@ -232,8 +233,9 @@ object TypedImperativeAggregateSuite {
       nullable: Boolean = false,
       mutableAggBufferOffset: Int = 0,
       inputAggBufferOffset: Int = 0)
-    extends TypedImperativeAggregate[MaxValue] with ImplicitCastInputTypes {
+    extends TypedImperativeAggregate[MaxValue]
-
+    with ImplicitCastInputTypes
+    with UnaryLike[Expression] {
 
     override def createAggregationBuffer(): MaxValue = {
       // Returns Int.MinValue if all inputs are null
@@ -270,8 +272,6 @@ object TypedImperativeAggregateSuite {
 
     override lazy val deterministic: Boolean = true
 
-    override def children: Seq[Expression] = Seq(child)
-
     override def inputTypes: Seq[AbstractDataType] = Seq(IntegerType)
 
     override def dataType: DataType = IntegerType

sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/TestingTypedCount.scala

@@ -22,6 +22,7 @@ import java.io.{ByteArrayInputStream, ByteArrayOutputStream, DataInputStream, Da
 import org.apache.spark.sql.catalyst.InternalRow
 import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.expressions.aggregate.{ImperativeAggregate, TypedImperativeAggregate}
+import org.apache.spark.sql.catalyst.trees.UnaryLike
 import org.apache.spark.sql.hive.execution.TestingTypedCount.State
 import org.apache.spark.sql.types._
 
@@ -32,12 +33,11 @@ case class TestingTypedCount(
     child: Expression,
     mutableAggBufferOffset: Int = 0,
     inputAggBufferOffset: Int = 0)
-  extends TypedImperativeAggregate[TestingTypedCount.State] {
+  extends TypedImperativeAggregate[TestingTypedCount.State]
+  with UnaryLike[Expression] {
 
   def this(child: Expression) = this(child, 0, 0)
 
-  override def children: Seq[Expression] = child :: Nil
-
   override def dataType: DataType = LongType
 
   override def nullable: Boolean = false