SELECT 
  COUNT(`product_review_id`) AS `count`, 
  `rating_value` 
FROM 
  `cscart_product_reviews` 
WHERE 
  `product_id` = 29136 
  AND `status` = 'A' 
GROUP BY 
  `rating_value`

Query time 0.00044

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.10"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": false,
      "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",
          "rating_value",
          "status"
        ],
        "attached_condition": "(`dev2`.`cscart_product_reviews`.`status` = 'A')"
      }
    }
  }
}