SELECT 
  feature_id, 
  variant_id, 
  value, 
  value_int 
FROM 
  cscart_product_features_values 
WHERE 
  product_id = 603 
  AND lang_code = 'en'

Query time 0.00040

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_features_values ref lang_code,product_id,product_id_idx,product_feature_idx product_id 3 const 38 Using where

Result

feature_id variant_id value value_int
127 5895
128 5899
129 5931
30 5933
46 0 150.000
121 0 700.000
122 0 0.020
33 0 1500.000
32 0 50000.000
178 6331