Correct subfield access logical when calling ITEM

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

core/src/main/java/org/opensearch/sql/ast/expression/QualifiedName.java

@@ -22,6 +22,7 @@
 @Getter
 @EqualsAndHashCode(callSuper = false)
 public class QualifiedName extends UnresolvedExpression {
+  public static final String DELIMITER = ".";
   private final List<String> parts;
 
   public QualifiedName(String name) {
@@ -94,7 +95,7 @@ public QualifiedName rest() {
   }
 
   public String toString() {
-    return String.join(".", this.parts);
+    return String.join(DELIMITER, this.parts);
   }
 
   @Override

core/src/main/java/org/opensearch/sql/calcite/QualifiedNameResolver.java

@@ -249,8 +249,10 @@ private static RexNode resolveFieldAccess(
     if (length == parts.size() - start) {
       return field;
     } else {
-      String itemName = joinParts(parts, length + start, parts.size() - 1 - length);
-      return createItemAccess(field, itemName, context);
+      String itemName = joinParts(parts, length + start, parts.size() - length);
+      return context.relBuilder.alias(
+          createItemAccess(field, itemName, context),
+          String.join(QualifiedName.DELIMITER, parts.subList(start, parts.size())));
     }
   }