SELECT 
  pfv.feature_id, 
  value 
FROM 
  cscart_product_features_values pfv 
  INNER JOIN cscart_product_features pf ON (pf.feature_id = pfv.feature_id) 
WHERE 
  pf.parent_id = 4 
  AND pfv.product_id = 18349 
  AND pfv.lang_code = 'en' 
  AND pf.status = 'A'

Query time 0.00088

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "18.51"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "pfv",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY",
            "fl",
            "lang_code",
            "product_id",
            "fpl",
            "idx_product_feature_variant_id"
          ],
          "key": "product_id",
          "used_key_parts": [
            "product_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 19,
          "rows_produced_per_join": 9,
          "filtered": "50.00",
          "index_condition": "(`cscart_db`.`pfv`.`lang_code` = 'en')",
          "cost_info": {
            "read_cost": "13.28",
            "eval_cost": "0.95",
            "prefix_cost": "15.18",
            "data_read_per_join": "7K"
          },
          "used_columns": [
            "feature_id",
            "product_id",
            "value",
            "lang_code"
          ]
        }
      },
      {
        "table": {
          "table_name": "pf",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY",
            "status"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "feature_id"
          ],
          "key_length": "3",
          "ref": [
            "cscart_db.pfv.feature_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 0,
          "filtered": "8.30",
          "cost_info": {
            "read_cost": "2.38",
            "eval_cost": "0.08",
            "prefix_cost": "18.51",
            "data_read_per_join": "353"
          },
          "used_columns": [
            "feature_id",
            "parent_id",
            "status"
          ],
          "attached_condition": "((`cscart_db`.`pf`.`parent_id` = 4) and (`cscart_db`.`pf`.`status` = 'A'))"
        }
      }
    ]
  }
}