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.
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.