SELECT 
  combination 
FROM 
  cscart_product_options_inventory 
WHERE 
  product_id = 9844 
  AND amount > 0 
  AND combination != ''

Query time 0.01410

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_options_inventory range pc pc 770 2376 Using index condition; Using where