SELECT 
  combination 
FROM 
  cscart_product_options_inventory 
WHERE 
  product_id = 10705 
  AND amount > 0 
  AND combination != ''

Query time 0.00039

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_options_inventory range pc pc 770 47 Using index condition; Using where

Result

combination
1295_23687
1295_23687_1293_23673
1295_23687_1293_23674
1295_23687_1293_23675
1295_23687_1293_23676
1295_23687_1293_23677
1295_23687_1293_23678
1295_23687_1293_23679
1295_23687_1293_23680
1295_23687_1293_23681
1295_23687_1293_23682
1295_23687_1293_23683