Cost estimation in foreign data wrappers

Hello,

There is a callback function in fdw's which should also set estimates for
startup and total costs for each path. Assume a fdw adds only one path
(e.g. in file_fdw). I am trying to understand what can go wrong if we do a
bad job in estimating these costs.

Since we have only one scan path here, it doesn't make a difference in
choosing the best scan path.

By looking at the code and doing some experiments, I think this can
be significant in (1) underestimating a nested loop's cost, (2) not
materializing inner table in nested loop.

* Are there any other cases that this can be significant?
* Assume we are not sure about the exact cost, but we know that it is in
[lower_bound, upper_bound] range, where upper_bound can be 10x lower_bound
Then, what value is better to choose? lower bound? upper bound? or average?

Thanks,
-- Hadi