Even better would be if the planner could estimate how bad a plan will become if we made assumptions that turn out to be wrong.
That's precisely what risk estimation was about.
Something like
SELECT * FROM wherever WHEN id > something LIMIT COST 10000;
Would forbid a sequential scan *if* the table is big enough to suspect the plan might take that much, or a nested loop *if* the planner cannot *prove* it will be faster than that.
I don't believe the limit unit is obscure at all (page fetches being a nice measuring stick), but what is, is what do you do when no plan fits the limits.
I don't think that's the same thing... what you're describing is a way to not begin a query if a low-enough cost plan can't be found.
What I'm talking about is when the planner picks one low-cost plan over another and it turns out the estimate of the one that was picked was WAY off. I've actually seen cases where plan estimates that were off by just 100 units produce wildly different results.
In that scenario, LIMIT COST won't help at all.
The case you describe is different. It's when a plan *effectively* is more expensive than estimated, but the planner could not estimate it.
That's what was mentioned about switching plans mid-way through them, which is IMO quite ill-defined as such (it needs a lot of love in order to get a workable spec out of that).
