To stretch it - I wonder if postresql is smart enough to recognize invariant subselects and keep those or if it runs the optimizer each time (in the case of sorted results). If it CAN recognize subselects, an additional layer of nesting might keep things fast (?)
e.g.
select a,b from foo where a=:a, b=:b order by $a_or_b;
select a,b from (select a,b from foo where a=:a, b=:b) order by $a_or_b;
