@classmethod
def format_sub_part(cls, field, length):
    """
        Generate the SQL to format the sub_part length of an indexed column name

          >>> schemaobjects.index.IndexSchema.format_sub_part('column', 0)
          '`column`'
          >>> schemaobjects.index.IndexSchema.format_sub_part('column', 5)
          '`column`(5)'
        """
    try:
        if not length:
            raise ValueError
        length = int(length)
        return '`%s`(%d)' % (field, length)
    except ValueError:
        return '`%s`' % (field,)
