pushing order by + limit to union subqueries

Hi all,
I've noticed that order by / limit are not distributed to union subqueries
by the planner:

Example:

q1: (select * from t1) union all (select * from t2) order by x limit 10;
q2: (select * from t1 order by x limit 10) union all (select * from t2
order by x limit 10)
order by x limit 10;

both queries should be equivalent, but the planner provides hugely different
plans. I was expecting that the planner could rewrite the first to the
second.
Am I overlooking something? If this is the case, can anyone explain why this
optimization is not performed?

Thanks!
Paolo