> but parallel processing doesn't requires threading support - see PostgreSQL 9.6 features.
To share dynamic execution code between threads much more easy(If sharing this code between process is possible). There is many other interaction techniques between threads which is absence between process.
We have shared memory.
How do you think the buffer cache works?
Lots more could be shared, too. Cached plans, for example.
It's possible. You just have to change how you think about it, and you might not be able to do it within the framework of the JVM. So it's quite possibly not worth it for you. Nonetheless, don't assume it can't be done just because you can't do it the way you're used to thinking of doing it.