On disable_cost

    Aside from not having a large enough disable cost, there's also the fact that the high cost might affect the rest of the plan, if we have to use a plan type that's disabled. For example, if a table doesn't have any indexes, but enable_seqscan is off, we might put the unavoidable Seq Scan on different side of a join than we we would with enable_seqscan=on, because of the high cost estimate.

I think a more robust way to disable forbidden plan types would be to handle the disabling in add_path(). Instead of having a high disable cost on the Path itself, the comparison add_path() would always consider disabled paths as more expensive than others, regardless of the cost.