SELECT 
  combination 
FROM 
  cscart_product_options_inventory 
WHERE 
  product_id = 1180 
  AND amount > 0 
  AND combination != ''

Query time 0.00042

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_options_inventory range pc pc 770 6 Using index condition; Using where

Result

combination
669_15556
669_15557
669_15558
669_15559
669_15560
669_15561
669_15562
669_15563