public static boolean isColumnOnly(VectorExpression ve) {
    if (ve instanceof IdentityExpression) {
      VectorExpression identityExpression = (IdentityExpression) ve;
      return (identityExpression.childExpressions == null);
    } else {
      return false;
    }
  }
An expression representing a column, only children are evaluated.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/IdentityExpression.java