def distinct_table(db, table, distinct_on):

    '''Creates a copy of a temp table which is distinct on the given columns.

    The table will get an index on these columns, facilitating merge joins.

    @param table Will be renamed to the distinct on table.

    @return The old table.

    '''

    if distinct_on == []: return table # already distinct

    old_table = copy.copy(table)

    table.name = sql_gen.concat(table.name, '_distinct')

    copy_table_struct(db, old_table, table)

    add_index(db, distinct_on, table, unique=True)

    insert_select(db, table, None, mk_select(db, old_table, start=0),

        ignore=True)

    analyze(db, table)

    return old_table