public boolean isCountStar() {
    if (!isAggregationFunctionExpression()) {
      return false;
    }
    return getPaths().size() == 1
        && paths.get(0) != null
        && (paths.get(0).getTailNode().equals(IoTDBConstant.ONE_LEVEL_PATH_WILDCARD)
            || paths.get(0).getTailNode().equals(IoTDBConstant.MULTI_LEVEL_PATH_WILDCARD))
        && functionName.equals(IoTDBConstant.COLUMN_COUNT);
  }
example: select udf(a, b, udf(c)) from root.sg.d;

<p>3 expressions [root.sg.d.a, root.sg.d.b, udf(root.sg.d.c)] will be in this field.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/expression/multi/FunctionExpression.java