Re-write query to perform

Question

I have a query which I have simplified below. The parts in bold retrieve the same resultset but are too expensive for me to execute twice. Is there a way to re-write this query so that the part in bold is only executed once?
I have tried using subquery factoring but that causes my <some_predicate> clause to be used at a later stage in the query which proves disastrous.
select * from (
select 'a' as cola, 1 as colb from dual
union all
select 'Total', sum(colb)
  from (select 'a' as cola, 1 as colb from dual union all
        select 'b' as cola, 2 as colb from dual)
) where <some_predicate>

Answer 1

How would you apply it to my example and get the same
output?
select decode(grouping_id(cola,colb,rownum),0, cola, 'TOTAL:') as cola,
       sum(colb) as colb
  from (
         select 'a' as cola, 1 as colb from dual union all
         select 'b' as cola, 2 as colb from dual
group by grouping sets((),(cola,colb,rownum))
having <some_predicate>