[SPARK-15692][SQL] Improves the explain output of several physical plans by displaying embedded logical plan in tree style

## What changes were proposed in this pull request?

Improves the explain output of several physical plans by displaying embedded logical plan in tree style

Some physical plan contains a embedded logical plan, for example, `cache tableName query` maps to:

```
case class CacheTableCommand(
    tableName: String,
    plan: Option[LogicalPlan],
    isLazy: Boolean)
  extends RunnableCommand
```

It is easier to read the explain output if we can display the `plan` in tree style.

**Before change:**

Everything is messed in one line.

```
scala> Seq((1,2)).toDF().createOrReplaceTempView("testView")
scala> spark.sql("cache table testView2 select * from testView").explain()
== Physical Plan ==
ExecutedCommand CacheTableCommand testView2, Some('Project [*]
+- 'UnresolvedRelation `testView`, None
), false
```

**After change:**

```
scala> spark.sql("cache table testView2 select * from testView").explain()
== Physical Plan ==
ExecutedCommand
:  +- CacheTableCommand testView2, false
:     :  +- 'Project [*]
:     :     +- 'UnresolvedRelation `testView`, None
```

## How was this patch tested?

Manual test.

Author: Sean Zhong <seanzhong@databricks.com>

Closes #13433 from clockfly/verbose_breakdown_3_2.

2 files changed, 8 insertions, 4 deletions

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 d4447ca32d..6784c3ae1d 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
@@ -264,7 +264,7 @@ abstract class QueryPlan[PlanType <: QueryPlan[PlanType]] extends TreeNode[PlanT
     expressions.flatMap(_.collect {case e: SubqueryExpression => e.plan.asInstanceOf[PlanType]})
   }
 
-  override def innerChildren: Seq[PlanType] = subqueries
+  override protected def innerChildren: Seq[QueryPlan[_]] = subqueries
 
   /**
    * Canonicalized copy of this query plan.
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala
index d87e6c76ed..3ebd815dce 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala
@@ -424,9 +424,12 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
    */
   protected def stringArgs: Iterator[Any] = productIterator
 
+  private lazy val allChildren: Set[TreeNode[_]] = (children ++ innerChildren).toSet[TreeNode[_]]
+
   /** Returns a string representing the arguments to this node, minus any children */
   def argString: String = productIterator.flatMap {
-    case tn: TreeNode[_] if containsChild(tn) => Nil
+    case tn: TreeNode[_] if allChildren.contains(tn) => Nil
+    case Some(tn: TreeNode[_]) if allChildren.contains(tn) => Nil
     case tn: TreeNode[_] => s"${tn.simpleString}" :: Nil
     case seq: Seq[BaseType] if seq.toSet.subsetOf(children.toSet) => Nil
     case seq: Seq[_] => seq.mkString("[", ",", "]") :: Nil
@@ -467,9 +470,10 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] extends Product {
   }
 
   /**
-   * All the nodes that are parts of this node, this is used by subquries.
+   * All the nodes that should be shown as a inner nested tree of this node.
+   * For example, this can be used to show sub-queries.
    */
-  protected def innerChildren: Seq[BaseType] = Nil
+  protected def innerChildren: Seq[TreeNode[_]] = Seq.empty
 
   /**
    * Appends the string represent of this node and its children to the given StringBuilder.