Insert/Update Upsert Trigger in Postgres

upsert.sql

CREATE OR REPLACE FUNCTION upsert_user()
  RETURNS trigger AS
$upsert_user$
declare
       existing record;
begin
     if (select EXISTS(select 1 from users where user_id  = NEW.user_id)) then

        select user_name, user_class, user_age into strict existing from users where user_id = new.user_id;

        --found; update, and return null to prevent insert
        UPDATE users SET 
            user_class = coalesce(new.user_class, existing.user_class),
            user_name = coalesce(new.user_name, existing.user_name),
            user_age = coalesce(new.user_age, existing.user_age),
            modified = current_timestamp
         WHERE user_id = new.user_id;
         
         return null;
     end if;
     
     new.modified = current_timestamp;
     return new;
end
$upsert_user$
LANGUAGE plpgsql;
  
create trigger users_insert
before insert
on users
for each row
execute procedure upsert_user();

Your first IF will lower the performance of your trigger. I think this can be much simpler. Thanks for the template.