The 9.5 release notes contain this promising but cryptic item:
- Allow foreign data wrappers and custom scans to implement join
pushdown (KaiGai Kohei)
As phrased, this seems to mean, "it can be done, but we haven't done
it". But there is no link to any documentation that explains how to do
this. The commit that appears to have added this feature touches
postgres_fdw, but neither the documentation nor the tests show anything
about this feature, and I haven't been able to reproduce anything that
would seem to indicate anything like this is at work.
So what is the actual extent of this feature?