[SPARK-17078][SQL][FOLLOWUP] Simplify explain cost command

## What changes were proposed in this pull request?

Usually when using explain cost command, users want to see the stats of plan. Since stats is only showed in optimized plan, it is more direct and convenient to include only optimized plan and physical plan in the output.

## How was this patch tested?

Enhanced existing test.

Author: Zhenhua Wang <wzh_zju@163.com>

Closes #18190 from wzhfy/simplifyExplainCost.

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

@@ -200,11 +200,7 @@ class QueryExecution(val sparkSession: SparkSession, val logical: LogicalPlan) {
       """.stripMargin.trim
   }
 
-  override def toString: String = completeString(appendStats = false)
-
-  def toStringWithStats: String = completeString(appendStats = true)
-
-  private def completeString(appendStats: Boolean): String = {
+  override def toString: String = {
     def output = Utils.truncatedString(
       analyzed.output.map(o => s"${o.name}: ${o.dataType.simpleString}"), ", ")
     val analyzedPlan = Seq(
@@ -212,25 +208,29 @@ class QueryExecution(val sparkSession: SparkSession, val logical: LogicalPlan) {
       stringOrError(analyzed.treeString(verbose = true))
     ).filter(_.nonEmpty).mkString("\n")
 
-    val optimizedPlanString = if (appendStats) {
-      // trigger to compute stats for logical plans
-      optimizedPlan.stats(sparkSession.sessionState.conf)
-      optimizedPlan.treeString(verbose = true, addSuffix = true)
-    } else {
-      optimizedPlan.treeString(verbose = true)
-    }
-
     s"""== Parsed Logical Plan ==
        |${stringOrError(logical.treeString(verbose = true))}
        |== Analyzed Logical Plan ==
        |$analyzedPlan
        |== Optimized Logical Plan ==
-       |${stringOrError(optimizedPlanString)}
+       |${stringOrError(optimizedPlan.treeString(verbose = true))}
        |== Physical Plan ==
        |${stringOrError(executedPlan.treeString(verbose = true))}
     """.stripMargin.trim
   }
 
+  def stringWithStats: String = {
+    // trigger to compute stats for logical plans
+    optimizedPlan.stats(sparkSession.sessionState.conf)
+
+    // only show optimized logical plan and physical plan
+    s"""== Optimized Logical Plan ==
+        |${stringOrError(optimizedPlan.treeString(verbose = true, addSuffix = true))}
+        |== Physical Plan ==
+        |${stringOrError(executedPlan.treeString(verbose = true))}
+    """.stripMargin.trim
+  }
+
   /** A special namespace for commands that can be used to debug query execution. */
   // scalastyle:off
   object debug {

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

@@ -127,7 +127,7 @@ case class ExplainCommand(
       } else if (extended) {
         queryExecution.toString
       } else if (cost) {
-        queryExecution.toStringWithStats
+        queryExecution.stringWithStats
       } else {
         queryExecution.simpleString
       }

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

@@ -29,6 +29,12 @@ class HiveExplainSuite extends QueryTest with SQLTestUtils with TestHiveSingleto
   import testImplicits._
 
   test("show cost in explain command") {
+    // For readability, we only show optimized plan and physical plan in explain cost command
+    checkKeywordsExist(sql("EXPLAIN COST  SELECT * FROM src "),
+      "Optimized Logical Plan", "Physical Plan")
+    checkKeywordsNotExist(sql("EXPLAIN COST  SELECT * FROM src "),
+      "Parsed Logical Plan", "Analyzed Logical Plan")
+
     // Only has sizeInBytes before ANALYZE command
     checkKeywordsExist(sql("EXPLAIN COST  SELECT * FROM src "), "sizeInBytes")
     checkKeywordsNotExist(sql("EXPLAIN COST  SELECT * FROM src "), "rowCount")