would it be a lot of work, to add optimizations accross unions ?

Say I have:

select foo ( select foo from bar1  union all select foo from bar2  union all select foo from bar3  ...
) a order by foo desc limit X;


(and I can give you few other examples around the same 'note', say  
with when foo=N in outer subselect)

Would anyone consider such optimization, when postgres will apply the  
same condition to inner queries, providing that their size is  
substantial?

Same would actually apply for different subqueries, without union/ 
intersect/etc:

select foo( select foo from bar1 ) a where foo in (x,y,z) order by foo  
desc limit N

(of course this is just a simplified example).