After insert trigger not work

Hello!
I use postgre-9.3.5 on windows7 x64.
Trigger should update data in table:
CREATE TABLE trassa.ram_free_stat
(
   id serial NOT NULL,
   device integer NOT NULL,
   min_value integer NOT NULL,
   avg_value integer NOT NULL DEFAULT 0,
   max_value integer NOT NULL,
   last_update timestamp without time zone NOT NULL DEFAULT now(),
   CONSTRAINT ram_free_stat_pk PRIMARY KEY (id),
   CONSTRAINT ram_free_stat_device_fk FOREIGN KEY (device)
       REFERENCES trassa.devices (id) MATCH SIMPLE
       ON UPDATE NO ACTION ON DELETE NO ACTION,
   CONSTRAINT ram_free_stat_max_fk FOREIGN KEY (max_value)
       REFERENCES trassa.ram (id) MATCH SIMPLE
       ON UPDATE NO ACTION ON DELETE NO ACTION,
   CONSTRAINT ram_free_stat_min_fk FOREIGN KEY (min_value)
       REFERENCES trassa.ram (id) MATCH SIMPLE
       ON UPDATE NO ACTION ON DELETE NO ACTION
)

My trigger:
CREATE OR REPLACE FUNCTION trassa.update_ram_free_stat()
   RETURNS trigger AS
$BODY$
DECLARE
device_id INTEGER DEFAULT 0;
min_id INTEGER DEFAULT 0;
avg_val INTEGER DEFAULT 0;
max_id INTEGER DEFAULT 0;
BEGIN
    SELECT id INTO device_id FROM trassa.ram_free_stat
        WHERE device = NEW.device;
    SELECT free_ram, id INTO min_id FROM trassa.ram
        WHERE device = NEW.device
        ORDER BY free_ram LIMIT 1;
    SELECT free_ram, id INTO max_id FROM trassa.ram
        WHERE device = NEW.device
        ORDER BY free_ram DESC LIMIT 1;
    SELECT CEIL(AVG(free_ram)) INTO avg_val
        FROM trassa.ram WHERE device = NEW.device;
    IF device_id > 0 THEN
        UPDATE trassa.ram_free_stat
            SET min_value = min_id,
                avg_value = avg_val,
                max_value = max_id
            WHERE id = device_id;
    ELSE
        INSERT INTO trassa.ram_free_stat
            (device, min_value,
                avg_value, max_value)
            VALUES(NEW.device, min_id,
                avg_val, max_id);
    END IF;
    RETURN NULL;
END;
$BODY$
   LANGUAGE plpgsql VOLATILE SECURITY DEFINER
   COST 100;

I add this trigger on another table:
CREATE TRIGGER update_ram_free_stat_trigger
   AFTER INSERT
   ON trassa.ram
   FOR EACH ROW
   EXECUTE PROCEDURE trassa.update_ram_free_stat();

All executed without any error, but data in trassa.ram_free_stat not
updated.
Can you help me solve this problem?
Thank you and excuse me for my bad english.

--
Best regards,
Brilliantov Kirill Vladimirovich