address comments.

sql/hive/src/main/scala/org/apache/spark/sql/hive/SQLBuilder.scala

@@ -107,9 +107,7 @@ class SQLBuilder(logicalPlan: LogicalPlan, sqlContext: SQLContext) extends Loggi
     case p: Project =>
       projectToSQL(p, isDistinct = false)
 
-    case a @ Aggregate(_, _, e @ Expand(_, _, p: Project))
-      if sameOutput(e.output,
-        p.child.output ++ a.groupingExpressions.map(_.asInstanceOf[Attribute])) =>
+    case a @ Aggregate(_, _, e @ Expand(_, _, p: Project)) if isGroupingSet(a, e, p) =>
       groupingSetToSQL(a, e, p)
 
     case p: Aggregate =>
@@ -208,10 +206,6 @@ class SQLBuilder(logicalPlan: LogicalPlan, sqlContext: SQLContext) extends Loggi
       throw new UnsupportedOperationException(s"unsupported plan $node")
   }
 
-  private def sameOutput(output1: Seq[Attribute], output2: Seq[Attribute]): Boolean =
-    output1.size == output2.size &&
-      output1.zip(output2).forall(pair => pair._1.semanticEquals(pair._2))
-
   /**
    * Turns a bunch of string segments into a single string and separate each segment by a space.
    * The segments are trimmed so only a single space appears in the separation.
@@ -242,6 +236,16 @@ class SQLBuilder(logicalPlan: LogicalPlan, sqlContext: SQLContext) extends Loggi
     )
   }
 
+  private def sameOutput(output1: Seq[Attribute], output2: Seq[Attribute]): Boolean =
+    output1.size == output2.size &&
+      output1.zip(output2).forall(pair => pair._1.semanticEquals(pair._2))
+
+  private def isGroupingSet(a: Aggregate, e: Expand, p: Project): Boolean = {
+    assert(a.child == e && e.child == p)
+    a.groupingExpressions.forall(_.isInstanceOf[Attribute]) &&
+      sameOutput(e.output, p.child.output ++ a.groupingExpressions.map(_.asInstanceOf[Attribute]))
+  }
+
   private def groupingSetToSQL(
       agg: Aggregate,
       expand: Expand,
@@ -253,16 +257,16 @@ class SQLBuilder(logicalPlan: LogicalPlan, sqlContext: SQLContext) extends Loggi
 
     val numOriginalOutput = project.child.output.length
     // Assumption: Aggregate's groupingExpressions is composed of
-    // 1) the group by attributes' aliases
+    // 1) the attributes of aliased group by expressions
     // 2) gid, which is always the last one
     val groupByAttributes = agg.groupingExpressions.dropRight(1).map(_.asInstanceOf[Attribute])
     // Assumption: Project's projectList is composed of
     // 1) the original output (Project's child.output),
-    // 2) the aliases of the original group by attributes, which could be expressions
+    // 2) the aliased group by expressions.
     val groupByExprs = project.projectList.drop(numOriginalOutput).map(_.asInstanceOf[Alias].child)
     val groupingSQL = groupByExprs.map(_.sql).mkString(", ")
 
-    // a map from the alias name to the original group by expresions/attributes
+    // a map from group by attributes to the original group by expressions.
     val groupByAttrMap = AttributeMap(groupByAttributes.zip(groupByExprs))
 
     val groupingSet = expand.projections.map { project =>