postgreSQL Triggers and Trigger Functions

CREATE OR REPLACE FUNCTION my_simple_trigger_function()
RETURNS trigger AS
$BODY$
BEGIN
 -- TG_TABLE_NAME :name of the table that caused the trigger invocation
IF (TG_TABLE_NAME = 'users') THEN
 --TG_OP : operation the trigger was fired
 IF (TG_OP = 'INSERT') THEN
 --NEW.id is holding the new database row value (in here id is the id column in users
table)
 --NEW will return null for DELETE operations
 INSERT INTO log_table (date_and_time, description) VALUES (now(), 'New user inserted. User
ID: '|| NEW.id);
 RETURN NEW;
 ELSIF (TG_OP = 'DELETE') THEN
 --OLD.id is holding the old database row value (in here id is the id column in users
table)
 --OLD will return null for INSERT operations
 INSERT INTO log_table (date_and_time, description) VALUES (now(), 'User deleted.. User ID:
' || OLD.id);
 RETURN OLD;
 END IF;
RETURN null;
END IF;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;

CREATE TRIGGER my_trigger
AFTER INSERT OR DELETE
ON users
FOR EACH ROW
EXECUTE PROCEDURE my_simple_trigger_function();