Oracle: GROUP BY extension ROLLUP

select
   case
     when grouping(owner) = 1 and grouping(segment_type) = 1 then '== GRAND TOTAL =='
     else                                                          owner
   end    owner_,
   case
     when grouping(owner) = 1 and grouping(segment_type) = 1 then  null
     when grouping(segment_type) = 1                         then '= TOTAL ='
     else                                                          segment_type
   end    type_,
   to_char(sum(bytes/1024/1024), '999,999,990.00') mb
from
   dba_segments
where
   owner in ('SYS', 'SYSTEM', user) and
   segment_type not in ('TYPE2 UNDO')
group by
   rollup(owner, segment_type)
order by
   owner,
   sum(bytes)
;