@Override
  public Identifier[] listFunctions(String[] namespace) throws NoSuchNamespaceException {
    TableCatalog tableCatalog = tableCatalog(TableFormat.ICEBERG);
    if (tableCatalog instanceof FunctionCatalog) {
      return ((FunctionCatalog) tableCatalog).listFunctions(namespace);
    }
    throw new NoSuchNamespaceException(namespace);
  }
List the functions in a namespace from the catalog.

<p>If there are no functions in the namespace, implementations should return an empty array.

@param namespace a multi-part namespace
@return an array of Identifiers for functions
@throws NoSuchNamespaceException If the namespace does not exist (optional).
amoro-format-mixed/amoro-mixed-spark/v3.2/amoro-mixed-spark-3.2/src/main/java/org/apache/amoro/spark/SparkUnifiedCatalog.java