When the tables are pretty big, the union operator last too long to retrieve the results.

Based on the structure you gave, you can try this:

select a.code, a.summt + b.summt as summt
from
(
select code, sum(amount) as SumMt
from tbla
group by code
) a,
(
select code, sum(amount) as SumMt
from tblb
group by code
) b
where a.code = b.code