When a PL/pgSQL function is called as a trigger, several special variables are 
created automatically in the top-level block. They are: 
NEW 
Data type RECORD; variable holding the new database row for INSERT/UPDATE 
operations in row-level triggers. This variable is NULL in statement-level triggers. 
OLD 
Data type RECORD; variable holding the old database row for UPDATE/DELETE 
operations in row-level triggers. This variable is NULL in statement-level triggers. 
TG_NAME 
Data type name; variable that contains the name of the trigger actually fired. 
TG_WHEN 
Data type text; a string of either BEFORE or AFTER depending on the trigger's 
definition. 
TG_LEVEL 
Data type text; a string of either ROW or STATEMENT depending on the trigger's 
definition. 
TG_OP 
Data type text; a string of INSERT, UPDATE, or DELETE telling for which operation the 
trigger was fired. 
TG_RELID 
Data type oid; the object ID of the table that caused the trigger invocation. 
TG_RELNAME 
Data type name; the name of the table that caused the trigger invocation. 
TG_NARGS 
Data type integer; the number of arguments given to the trigger procedure in the 
CREATE TRIGGER statement. 
TG_ARGV[] 
Data type array of text; the arguments from the CREATE TRIGGER statement. The 
index counts from 0. Invalid indices (less than 0 or greater than or equal to tg_nargs) 
result in a null value. 

CREATE TABLE emp (
    empname text,
    salary integer,
    last_date timestamp,
    last_user text
);

CREATE FUNCTION emp_stamp() RETURNS trigger AS $emp_stamp$
    BEGIN
        -- Check that empname and salary are given
        IF NEW.empname IS NULL THEN
            RAISE EXCEPTION 'empname cannot be null';
        END IF;
        IF NEW.salary IS NULL THEN
            RAISE EXCEPTION '% cannot have null salary', NEW.empname;
        END IF;

        -- Who works for us when she must pay for it?
        IF NEW.salary <  0  THEN
            RAISE EXCEPTION '% cannot have a negative salary', NEW.empname;
        END IF;

        -- Remember who changed the payroll when
        NEW.last_date := current_timestamp;
        NEW.last_user := current_user;
        RETURN NEW;
    END;
$emp_stamp$ LANGUAGE plpgsql;

CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp
    FOR EACH ROW EXECUTE PROCEDURE emp_stamp();