SELECT 
  feature_id, 
  variant_id, 
  value, 
  value_int 
FROM 
  cscart_product_features_values 
WHERE 
  product_id = 8399 
  AND lang_code = 'en'

Query time 0.00069

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "17.85"
    },
    "table": {
      "table_name": "cscart_product_features_values",
      "access_type": "ref",
      "possible_keys": [
        "lang_code",
        "product_id",
        "idx_product_feature_variant_id"
      ],
      "key": "product_id",
      "used_key_parts": [
        "product_id"
      ],
      "key_length": "3",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 51,
      "rows_produced_per_join": 16,
      "filtered": "32.52",
      "cost_info": {
        "read_cost": "12.75",
        "eval_cost": "1.66",
        "prefix_cost": "17.85",
        "data_read_per_join": "12K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "value",
        "value_int",
        "lang_code"
      ],
      "attached_condition": "(`softwarepirmam_hewadelivard_cscart_4`.`cscart_product_features_values`.`lang_code` = 'en')"
    }
  }
}