def _get_columns_relevant_for_diff(columns_to_show):
    """
    Extract columns that are relevant for the diff table.

    @param columns_to_show: (list) A list of columns that should be shown
    @return: (set) Set of columns that are relevant for the diff table. If
             none is marked relevant, the column named "status" will be
             returned in the set.
    """
    cols = {col.title for col in columns_to_show if col.relevant_for_diff}
    if len(cols) == 0:
        return {col.title for col in columns_to_show if col.title == "status"}
    else:
        return cols