Re: Cost overestimation of foreign JOIN

Andrey Lepikhov <a(dot)lepikhov(at)postgrespro(dot)ru> writes:
> Maybe it is needed to swap lines 2908 and 2909 (see attachment)?

No; as explained in the comment immediately above here, we're assuming
that the join conditions will be applied on the cross product of the
input relations.

Now admittedly, that's a worst-case assumption, since it amounts to
expecting that the remote server will do the join in the dumbest
possible nested-loop way. If the remote can use a merge or hash
join, for example, the cost is likely to be a lot less. But it is
not the job of this code path to outguess the remote planner. It's
certainly not appropriate to invent an unprincipled cost estimate
as a substitute for trying to guess that.

If you're unhappy with the planning results you get for this,
why don't you have use_remote_estimate turned on?

regards, tom lane