MYSQL_FDW trigger BEFORE UPDATE changes to NEW on a col not in the update statement don't go through

From: Francois Payette <francoisp(at)netmosphere(dot)net>
To: pgsql-hackers(at)postgresql(dot)org
Subject: MYSQL_FDW trigger BEFORE UPDATE changes to NEW on a col not in the update statement don't go through
Date: 2020-04-21 23:09:03
Message-ID: A38DD5ED-0DD3-4CCA-9A64-08CE10675344@netmosphere.net
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Hi All,
I was pleasantly surprised to see that triggers can be created on FDW tables. I'm running into a problem.

I create a trigger on an imported foreign table. In the procedure, I change the value of a column that is not in the triggering update statement. This change does not make it to the mysql side.

CREATE OR REPLACE FUNCTION aatrigger_up() returns trigger
AS $$
DECLARE
BEGIN

IF NOT(row_to_json(NEW)->'pgrti' is NULL) THEN
NEW.pgrti = 2000000000*random();
END IF;
RAISE NOTICE 'aarigger_up %', row_to_json(NEW)::text;
return NEW;

END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER aarigger_up BEFORE UPDATE ON mysql.users FOR EACH ROW EXECUTE PROCEDURE aarigger_up();
update mysql.users set email = 'admin(at)example(dot)com' where id = 1;
I can see that the value for pgrti is updated in the NOTICE in postgres. In mysql the value is not updated. If I add the target col to the statement it does go through

update mysql.users set email = 'admin(at)example(dot)com', pgrti=0 where id = 1;
I need this to work to be able to detect CRUD coming from PG in a little deamon that calls pg_triggers for updates coming from mysqld; without a means to detect changes originating from pg the triggers would fire twice. Any idea where I'd change MYSQL_FDW to do this (also add fields that are updated in the trigger before firing off to mysql)?

I’m seeing in https://github.com/EnterpriseDB/mysql_fdw/blob/master/deparse.c <https://github.com/EnterpriseDB/mysql_fdw/blob/master/deparse.c> in
mysql_deparse_update

That the actual update statement is used to generate the mapping, so any col referred to in triggers would be ignored…

TIA, stay safe!
Francois Payette

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