SQL query optimization in Prestashop e-Commerce solution

I'm working on a layered navigation module within Prestashop e-Commerce solution. I'm trying to optimize a SQL query which creates a temp table during execution. I think this is creating a bottleneck in MySQL. This is the query: SELECT p.*, p.id_category_default, pl.available_later, pl.description_short, pl.link_rewrite, pl.name, i.id_image, il.legend, m.name manufacturer_name, DATEDIFF(p.date_add, DATE_SUB(NOW(), INTERVAL 30 DAY)) > 0 AS new FROM ps_category_product cp LEFT JOIN ps_category c ON (c.id_category = cp.id_category) LEFT JOIN ps_product p ON p.id_product = cp.id_product LEFT JOIN ps_product_lang pl ON (pl.id_product = p.id_product) LEFT JOIN ps_image i ON (i.id_product = p.id_product AND i.cover = 1) LEFT JOIN ps_image_lang il ON (i.id_image = il.id_image AND il.id_lang = 2) LEFT JOIN ps_manufacturer m ON (m.id_manufacturer = p.id_manufacturer) WHERE p.active = 1 AND c.nleft >= 2 AND c.nright <= 29 AND c.active = 1 AND pl.id_lang = 2 AND p.id_product IN (74, 78, 130, 146, 168, 169, 178, 195, ..., 297, 302, 1986, 1987, 1988, 1993, 1999, 2000, 2001) GROUP BY p.id_product ORDER BY p.date_upd desc LIMIT 0,48 I know the temp table is created because of the GROUP BY but I don't know how to remove it. If the same process could be done with PHP instead of SQL, that could be a solution.