Comments - CREATE FUNCTION

Don't understand in which cases function above will be work incorrect, because I defined it as "DETERMINISTIC", but it really READ SQL DATA.

DELIMITER $$
CREATE DEFINER=`root`@`localhost` FUNCTION `bpl_get_product_by_code`(
par_product_code VARCHAR(10)
) RETURNS VARBINARY(36)
    DETERMINISTIC
BEGIN
	DECLARE var_id_product VARBINARY(36);	
	SELECT id_product INTO var_id_product
	FROM bpl_product_types
	WHERE `code_product` = par_product_code;
	RETURN var_id_product;
    END$$

DELIMITER ;

If I defined it as "READ SQL DATA" follow SQL query

SELECT
  bpl_product_prices.id_product code_product
  ,ext_code code_shop
  ,SUM(cnt) cnt
  ,price
FROM bpl_product_prices
JOIN bpl_departments_stores USING (id_depart)
JOIN bpl_product_cnts USING (id_store, id_product)
JOIN `#departments` USING (id_depart)
WHERE
bpl_product_prices.id_product = bpl_get_product_by_code('15693')
AND (bpl_product_prices.id_depart = get_id_depart('') OR ('' = ''))
AND price > 0
AND cnt > 0
GROUP BY id_depart

will executing above 2 min.