SELECT 
  feature_id, 
  variant_id, 
  value, 
  value_int 
FROM 
  cscart_product_features_values 
WHERE 
  product_id = 523 
  AND lang_code = 'en'

Query time 0.00046

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 78 Using where

Result

feature_id variant_id value value_int
152 6095
152 6092
178 6317
30 6131
127 5895
128 5899
129 5902
42 145
22 98
23 6273
21 95
20 6052 1.300
59 5876 3.000
24 107
152 6094
134 6238
152 6089
152 6088
152 6096
152 6105
19 106 1.000