Optimizer/Planner levels

Hi,

I was wondering if there is a way the set levels,
or otherwize influence the behaviour of the optimizer.
In the near future I will need to do some very large joins
of very large tables.
I have reason to believe that the final resul will be manageable,
but I do not have definite information about the optimal join order.
Without very aggressive optimization,
it is likely the intermediate results will swamp the system.
I would be willing to spend minutes, even hours, in the optyimizer
for the sake of getting a feasible query plan.
Is there a way to tell the optimizer to just do whatever it takes.
Also is there a way to give it partial heuristic advice
on constructing the query.

thanks
Daniel