The problem, though, is that it won't ALWAYS be right to implement OR using UNION, so you have to have some way of deciding which is better.
That's easy - you propose both ways to planner and it's up to it to decide. Yes, it can decide wrong way, but we are returning to statistics problem. At least one can tune costs and enable_ settings. Now one have to rewrite query that may be not possible/too complex.
