[tempguy9999]

Perhaps, as a learning aid, and definitely to help fix erroneous query plans, but you'd lose the ability for the DB to adapt to new situations such as added data, data distributions changing, the addition/removal of indexes etc.

Also it's downright hard to make a good query plan for a nontrivial query, and it gets exponentially harder.

Honestly the optimiser is in a better position to do the work.

On re-reading I think you're talking only about pedagogy in which case disregard the list-of-cons in my post. And I think it would be a damn good idea! I really like it.