Postgresql trigger function with parameters

Question

I want to create a trigger on a table called takes in postgresql to update a value in another table called student I'm trying to do it in the following way. But I'm getting an error that there is syntax error near "OLD". I don't understand whats wrong with this. This is my code:

CREATE OR REPLACE FUNCTION upd8_cred_func
      (id1 VARCHAR, gr1 VARCHAR,id2 VARCHAR, gr2 VARCHAR) 
      RETURNS void AS $$
 BEGIN
    IF  (id1=id2 and gr1 is null and gr2 is not null) THEN 
        update student set tot_cred = tot_cred + 6 where id = id1;
    END IF;
    RETURN;
 END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER upd8_cred
    AFTER UPDATE ON takes
    FOR EACH ROW
    EXECUTE PROCEDURE upd8_cred_func(OLD.id,OLD.grade,NEW.id,NEW.grade);

Answer 1

You do not need to pass the NEW and OLD as parameters to the trigger function. They are automagically available there:

http://www.postgresql.org/docs/9.1/interactive/trigger-definition.html :

The trigger function must be declared as a function taking no arguments and returning type trigger. (The trigger function receives its input through a specially-passed TriggerData structure, not in the form of ordinary function arguments.)

About the records passed to the trigger procedure, please see http://www.postgresql.org/docs/9.1/interactive/plpgsql-trigger.html :

When a PL/pgSQL function is called as a trigger, several special variables are created automatically in the top-level block. They are: [...] NEW, [...] OLD [...]

As SeldomNeedy pointed in the comment below, you can still pass and use parameters to the trigger function. You declare the function as taking no parameters, but when defining the trigger (by CREATE TRIGGER), you may add some.

They will be available for the trigger as TG_NARG (the number of such parameters), and TG_ARGV[] (an array of text values).