How to use variable settings in trigger functions?

Question

I would like to record the id of a user in the session/transaction, using SET, so I could be able to access it later in a trigger function, using current_sett

Answer 1

It is not clear why you are trying to concat NULL to user_id but it is obviously the cause of the problem. Get rid of it:

CREATE OR REPLACE FUNCTION add_transition1() RETURNS TRIGGER AS $$
    DECLARE
        user_id integer;
    BEGIN
        user_id := current_setting('myvars.user_id')::integer;
        INSERT INTO transitions1 (user_id, house_id) VALUES (user_id, NEW.id);
        RETURN NULL;
    END;
$$ LANGUAGE plpgsql;

Note that

SELECT 55 || NULL

always gives NULL.