Re: disfavoring unparameterized nested loops

> In general I suspect that we'd be better off focussing on mitigating
> the impact at execution time. There are at least a few things that we
> could do there, at least in theory. Mostly very ambitious, long term
> things.

I think these things are orthogonal. No matter how good the cost model ever gets, we will always have degenerate cases.
Having some smarts about that in the executor is surely a good thing, but it shouldn't distract us from improving on the planner front.

>
> I like the idea of just avoiding unparameterized nested loop joins
> altogether when an "equivalent" hash join plan is available because
> it's akin to an execution-time mitigation, despite the fact that it
> happens during planning. While it doesn't actually change anything in
> the executor, it is built on the observation that we have virtually
> everything to gain and nothing to lose during execution, no matter
> what happens.

I agree with you, that those plans are too risky. But let's maybe find a more general way of dealing with this.

> Right. Though I am actually sympathetic to the idea that users might
> gladly pay a cost for performance stability -- even a fairly large
> cost. That part doesn't seem like the problem.