[TylerE]

I don't think it's sop muhch a matter of "expensive" as "unknowable".

For instance, if you have a left join (and let's say it can't use an index for whatever reason) - the optimal plan will probably be different if almost every row has a matching row(s) in the join table than if only a few do.

[davidrowley]

I think the join search would remain at the same level of exhaustiveness for all levels of optimisation. I imagined we'd maybe want to disable optimisations that apply more rarely or are most expensive to discover when the planner "optimisation level" was set to lower settings. I suppose that would be things like LEFT JOIN removals and self-join removals. However, PostgreSQL does not have very many expensive optimisations that rarely help, so having an optimisation level might be more of a way of introducing more new optimisations that help fewer queries. Because we don't have a plan cache, there's been a focus on keeping the planner lean and having it not go to too much effort to optimise queries that are poorly written, for example.