@Override
  public ExpressionType getExpressionType() {
    return ExpressionType.FUNCTION;
  }
Generates the parameter part of the function column name.

<p>Example:

<p>Full column name -> udf(root.sg.d.s1, sin(root.sg.d.s1), 'key1'='value1', 'key2'='value2')

<p>The parameter part -> root.sg.d.s1, sin(root.sg.d.s1), 'key1'='value1', 'key2'='value2'
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/expression/multi/FunctionExpression.java