SELECT 
  combination 
FROM 
  cscart_product_options_inventory 
WHERE 
  product_id = 10967 
  AND amount > 0 
  AND combination != ''

Query time 0.00057

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_options_inventory range pc pc 770 162 Using index condition; Using where

Result

combination
1323_24005_1322_23999
1323_24005_1322_24000
1323_24005_1322_24001
1323_24005_1322_24002