use fieldNames to preserve order

Author: xysun

sql/catalyst/src/main/scala/org/apache/spark/sql/types/StructType.scala

@@ -273,7 +273,7 @@ case class StructType(fields: Array[StructField]) extends DataType with Seq[Stru
     nameToField.getOrElse(name,
       throw new IllegalArgumentException(
         s"""Field "$name" does not exist.
-           |Available fields: ${fieldNamesSet.mkString(", ")}""".stripMargin))
+           |Available fields: ${fieldNames.mkString(", ")}""".stripMargin))
   }
 
   /**
@@ -287,7 +287,7 @@ case class StructType(fields: Array[StructField]) extends DataType with Seq[Stru
     if (nonExistFields.nonEmpty) {
       throw new IllegalArgumentException(
         s"""Nonexistent field(s): ${nonExistFields.mkString(", ")}.
-           |Available fields: ${fieldNamesSet.mkString(", ")}""".stripMargin)
+           |Available fields: ${fieldNames.mkString(", ")}""".stripMargin)
     }
     // Preserve the original order of fields.
     StructType(fields.filter(f => names.contains(f.name)))
@@ -302,7 +302,7 @@ case class StructType(fields: Array[StructField]) extends DataType with Seq[Stru
     nameToIndex.getOrElse(name,
       throw new IllegalArgumentException(
         s"""Field "$name" does not exist.
-           |Available fields: ${fieldNamesSet.mkString(", ")}""".stripMargin))
+           |Available fields: ${fieldNames.mkString(", ")}""".stripMargin))
   }
 
   private[sql] def getFieldIndex(name: String): Option[Int] = {