SELECT 
  feature_id, 
  variant_id, 
  value, 
  value_int 
FROM 
  cscart_product_features_values 
WHERE 
  product_id = 406 
  AND lang_code = 'en'

Query time 0.00039

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 59 Using where

Result

feature_id variant_id value value_int
176 6260
127 5895
128 5899
129 5902
30 128
42 148
135 5965
178 6306
135 5970
135 5969
24 108
135 5966
176 6270
176 6262
135 5968
135 5964