def extract_field_names(dict_list):
    """ Returns a sorted list of field names from a dictionary list
    > extract_field_names([{'a': 1, 'b': 2}, {'a': 3, 'c': 4}])
    ['a', 'b', 'c']
    """
    field_names = []
    for row_dict in dict_list:
        field_names += row_dict.keys()
    field_names = list(set(field_names))  # Removes duplicates
    return sorted(field_names)