add doc

Author: cloud-fan

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

@@ -97,6 +97,9 @@ object ExtractValue {
  * Returns the value of fields in the Struct `child`.
  *
  * No need to do type checking since it is handled by [[ExtractValue]].
+ *
+ * Note that we can pass in the field name directly to keep case preserving in `toString`.
+ * For example, when get field `yEAr` from `<year: int, month: int>`, we should pass in `yEAr`.
  */
 case class GetStructField(child: Expression, ordinal: Int, name: Option[String] = None)
   extends UnaryExpression {