After insert trigger not work

[Brilliantov Kirill Vladimirovich <brilliantov@xxxxxxxxx>]

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



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