SELECT 
  feature_id, 
  variant_id, 
  value, 
  value_int 
FROM 
  cscart_product_features_values 
WHERE 
  product_id = 515 
  AND lang_code = 'en'

Query time 0.00037

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 37 Using where

Result

feature_id variant_id value value_int
174 6232
127 5895
128 5899
129 5902
30 6223
171 6225
169 0 1.310
173 6228
170 6219
178 6316