Generally, I'd say the idea of deleting and inserting into a summation table in a trigger on a detail table is something that works well only with maximum one concurrent user per row of the summation table. It simply contradicts concurrency!

What I'd rather recommend, is that the trigger always inserts into the summation table, but never deletes from it. If you insert a new record into twarehouse, the trigger should insert +1 into twarehouse_sum, if you delete a record from twqarehouse, the trigger should insert -1 into twarehouse_sum (change +1/-1 appropriately if the summation table contains sums rather than counts). Then you should have a separate process that at set times (e.g. once every hour, day or week), does something like:

for select DistinctField, sum(MySummationField) from twarehouse_sum group by 1

    into :DistinctField, :MySum do

begin

  delete from twarehouse_sum where DistinctField = :DistinctField;

  insert into twarehouse_sum(DistinctField, SummationField)

  values(:DistinctField, :MySum);

end

That would require you to generally change all your

  select DistinctField, MySummationField

  from twarehouse_sum

to 

  select DistinctField, sum(MySummationField)

  from twarehouse_sum

  group by 1

whenever you want to find the sum, but it is scalable (i.e. a lot better for concurrency) and avoids the problem you've observed.

Set