BUG #15315: Trigger on foreign data wrapper (MYSQL) does not update columns not in set clause

The following bug has been logged on the website:

Bug reference: 15315
Logged by: Francesco Pirotti
Email address: fpirotti@gmail.com
PostgreSQL version: 9.6.7
Operating system: Debian Linux 8
Description:

I have a foreign table "t1" with two columns, "A" and "B" from mysql_fdw
foreign data wrapper.
I have created a trigger so that column B gets automatically a value when A
is updated
BEGIN
NEW.B := 'dummy value';
RETURN NEW;
END;

if I run the following query
update t1 set A='1'
column B does NOT update as per trigger.
if I run the following query
update t1 set A='1', B='a'
then column B correctly gets the 'dummy value' from the trigger function.

I created an identical table in postgresql (create table t2 as select * from
t1), so it is not foreign, and both updates work!

Thanks for your attention: PostgreSQL 9.6.7 on x86_64-pc-linux-gnu (Debian
9.6.7-1.pgdg80+1), compiled by gcc (Debian 4.9.2-10) 4.9.2, 64-bit