@Override
  public boolean isConstantOperandInternal() {
    if (isConstantOperandCache == null) {
      isConstantOperandCache = true;
      for (Expression inputExpression : expressions) {
        if (!inputExpression.isConstantOperand()) {
          isConstantOperandCache = false;
          break;
        }
      }
    }
    return isConstantOperandCache;
  }
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