improving windows functions performance

Hey,

I have a very big query that consist from 3-4 subqueries that use windows functions. There is a chance that I'll need to rewrite the query but first I'm trying to search for other ways to improve it and I'll be happy to hear if one of u have an idea.

Basically my table has the following structure : (objid,first_num,last_num) and each record is a range from the first number to the last one for that specific obj. I'm trying to unite ranges that overlaps. For example : 

for the following table : 

objid first_num last_num

1          5                7

1          8                 10

2          4                   6

2          9                   10

I would like to get :