I think you could probably get hints into Postgres if you did them right (and volunteered to do the work!). Where "right" means trying to make the hints at least somewhat declarative: not "use index X for query Y", but "the values of these two attributes are not independent" or "this predicate is likely to be highly selective." Basically, look at the most frequent planner failures, figure out why the planner makes a mistake, and let the user specify the information the planner needs to choose the right plan. Postgres has added features in this direction: for example, you can now specify the cost associated with evaluating a function as part of CREATE FUNCTION.
The work required to implement this would be substantial, and most of the people with the necessary skills would rather improve the planner itself (e.g., collect statistics on cross-column correlations to avoid making the attribute independence assumption in the first place). So it isn't too surprising this hasn't got done.
The work required to implement this would be substantial, and most of the people with the necessary skills would rather improve the planner itself (e.g., collect statistics on cross-column correlations to avoid making the attribute independence assumption in the first place). So it isn't too surprising this hasn't got done.