SELECT 
  (
    SUM(rating_value) / COUNT(product_review_id)
  ) AS rating 
FROM 
  `cscart_product_reviews` 
WHERE 
  `product_id` = 29136 
  AND `reply_id` = 0 
  AND `status` = 'A'

Query time 0.00034

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.10"
    },
    "table": {
      "table_name": "cscart_product_reviews",
      "access_type": "ref",
      "possible_keys": [
        "idx_product_id"
      ],
      "key": "idx_product_id",
      "used_key_parts": [
        "product_id"
      ],
      "key_length": "3",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 1,
      "rows_produced_per_join": 0,
      "filtered": "33.33",
      "cost_info": {
        "read_cost": "1.00",
        "eval_cost": "0.03",
        "prefix_cost": "1.10",
        "data_read_per_join": "560"
      },
      "used_columns": [
        "product_review_id",
        "product_id",
        "reply_id",
        "rating_value",
        "status"
      ],
      "attached_condition": "((`dev2`.`cscart_product_reviews`.`reply_id` = 0) and (`dev2`.`cscart_product_reviews`.`status` = 'A'))"
    }
  }
}