SELECT 
  combination 
FROM 
  cscart_product_options_inventory 
WHERE 
  product_id = 10070 
  AND amount > 0 
  AND combination != ''

Query time 0.00051

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_options_inventory range pc pc 770 7 Using index condition; Using where

Result

combination
614_15104
614_15105
614_15106
614_15107
614_15108
614_15109
614_15110
614_15111
614_15112
614_15113
614_15114
614_15115
614_15553
614_15554