Re: Cost overestimation of foreign JOIN

On 30.11.2020 22:38, Tom Lane wrote:
> 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.

Thank you. Now it is clear to me.
>
> 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.

My goal is scaling Postgres on a set of the same servers with same
postgres instances. If one server uses for the join a hash-join node, i
think it is most likely that the other server will also use for this
join a hash-join node (Maybe you remember, I also use the statistics
copying technique to provide up-to-date statistics on partitions). Tests
show good results with such an approach. But maybe this is my special case.

> 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.

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

I have a mixed load model. Large queries are suitable for additional
estimate queries. But for many simple SELECT's that touch a small
portion of the data, the latency has increased significantly. And I
don't know how to switch the use_remote_estimate setting in such case.

--
regards,
Andrey Lepikhov
Postgres Professional