> What happens if someone changes the record on the foreign side between when
> we've read it and we do the UPDATE?
>
Concurrency control is job of FDW driver. It has to coordinate access to
the records to be fetched for update / delete.
In fact, postgres_fdw add "FOR UPDATE" to avoid concurrent update
when it issues 1st-stage query to remote server.
Thanks,
--
KaiGai Kohei <kaigai@kaigai.gr.jp>