SELECT 
  rating_value, 
  COUNT(1) count 
FROM 
  cscart_product_reviews 
WHERE 
  `product_id` = 3976 
  AND `status` = 'A' 
GROUP BY 
  rating_value

Query time 0.00895

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "8.80"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": false,
      "table": {
        "table_name": "cscart_product_reviews",
        "access_type": "index_merge",
        "possible_keys": [
          "idx_product_id",
          "idx_product_reviews_status"
        ],
        "key": "intersect(idx_product_id,idx_product_reviews_status)",
        "key_length": "3,3",
        "rows_examined_per_scan": 6,
        "rows_produced_per_join": 6,
        "filtered": "92.31",
        "cost_info": {
          "read_cost": "8.20",
          "eval_cost": "0.60",
          "prefix_cost": "8.80",
          "data_read_per_join": "9K"
        },
        "used_columns": [
          "product_review_id",
          "product_id",
          "rating_value",
          "status"
        ],
        "attached_condition": "((`cscart_db`.`cscart_product_reviews`.`product_id` = 3976) and (`cscart_db`.`cscart_product_reviews`.`status` = 'A'))"
      }
    }
  }
}