SELECT 
  product_id 
FROM 
  cscart_product_variation_group_products 
WHERE 
  group_id IN (12)

Query time 0.00027

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_variation_group_products ref idx_group_id idx_group_id 3 const 4

Result

product_id
162
290
291
292