PL/SQL :: Oracle Version Is 10g - Bulk Update?

My oracle version is 10g.I want to update a table which has 2 million rows and 20 columns. I want to update 3 columns in my table based on a query output.

Say:-
select decode(col1,'a',col2,col3) up_col1, decode(col5,'Y',20,30)* col6 up_col2, col7 up_col3 from base_tab a, update_tab b where a.key = b.key

I tried the simple update SQL, it hangs and never comeback even after 2 hrs.So I resort to PLSQL to complete my job using bulk collect with limits. Now I am stumbled upon with the Bulk update step.I referred to the BULK Update example given in [URL]...

I gone by 9i approach, I got invalid rowid error. Since the example doesn't uses a join.

I am little bit worried to go by 10g approach given in the example. I have to fetch entire row from my update table. My update table has 5 varchar(1000) fields. Anyway I am not going to affect them in my update. Whether it will occupy memory and again leads to hanging of my procedure.