[SQL][minor] remove duplicated resolveGetField and update comment

It's after #5189

Author: Wenchen Fan <cloud0fan@outlook.com>

Closes #5304 from cloud-fan/tmp and squashes the following commits:

c58c9b3 [Wenchen Fan] remove duplicated code and update comment

Author: cloud-fan

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala

@@ -293,7 +293,7 @@ class Analyzer(
             logDebug(s"Resolving $u to $result")
             result
           case UnresolvedGetField(child, fieldName) if child.resolved =>
-            resolveGetField(child, fieldName)
+            q.resolveGetField(child, fieldName, resolver)
         }
     }
 
@@ -313,36 +313,6 @@ class Analyzer(
      */
     protected def containsStar(exprs: Seq[Expression]): Boolean =
       exprs.exists(_.collect { case _: Star => true }.nonEmpty)
-
-    /**
-     * Returns the resolved `GetField`, and report error if no desired field or over one
-     * desired fields are found.
-     */
-    protected def resolveGetField(expr: Expression, fieldName: String): Expression = {
-      def findField(fields: Array[StructField]): Int = {
-        val checkField = (f: StructField) => resolver(f.name, fieldName)
-        val ordinal = fields.indexWhere(checkField)
-        if (ordinal == -1) {
-          throw new AnalysisException(
-            s"No such struct field $fieldName in ${fields.map(_.name).mkString(", ")}")
-        } else if (fields.indexWhere(checkField, ordinal + 1) != -1) {
-          throw new AnalysisException(
-            s"Ambiguous reference to fields ${fields.filter(checkField).mkString(", ")}")
-        } else {
-          ordinal
-        }
-      }
-      expr.dataType match {
-        case StructType(fields) =>
-          val ordinal = findField(fields)
-          StructGetField(expr, fields(ordinal), ordinal)
-        case ArrayType(StructType(fields), containsNull) =>
-          val ordinal = findField(fields)
-          ArrayGetField(expr, fields(ordinal), ordinal, containsNull)
-        case otherType =>
-          throw new AnalysisException(s"GetField is not valid on fields of type $otherType")
-      }
-    }
   }
 
   /**

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/LogicalPlan.scala

@@ -205,11 +205,10 @@ abstract class LogicalPlan extends QueryPlan[LogicalPlan] with Logging {
       // One match, but we also need to extract the requested nested field.
       case Seq((a, nestedFields)) =>
         try {
-
-          // The foldLeft adds UnresolvedGetField for every remaining parts of the name,
-          // and aliased it with the last part of the name.
-          // For example, consider name "a.b.c", where "a" is resolved to an existing attribute.
-          // Then this will add UnresolvedGetField("b") and UnresolvedGetField("c"), and alias
+          // The foldLeft adds GetFields for every remaining parts of the identifier,
+          // and aliases it with the last part of the identifier.
+          // For example, consider "a.b.c", where "a" is resolved to an existing attribute.
+          // Then this will add GetField("c", GetField("b", a)), and alias
           // the final expression as "c".
           val fieldExprs = nestedFields.foldLeft(a: Expression)(resolveGetField(_, _, resolver))
           val aliasName = nestedFields.last
@@ -234,10 +233,8 @@ abstract class LogicalPlan extends QueryPlan[LogicalPlan] with Logging {
   /**
    * Returns the resolved `GetField`, and report error if no desired field or over one
    * desired fields are found.
-   *
-   * TODO: this code is duplicated from Analyzer and should be refactored to avoid this.
    */
-  protected def resolveGetField(
+  def resolveGetField(
       expr: Expression,
       fieldName: String,
       resolver: Resolver): Expression = {