SELECT 
  rating_value, 
  COUNT(1) count 
FROM 
  cscart_product_reviews 
WHERE 
  `product_id` = 309 
  AND `status` = 'A' 
GROUP BY 
  rating_value

Query time 0.00019

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_reviews",
          "access_type": "system",
          "possible_keys": ["idx_product_id"],
          "rows": 1,
          "filtered": 100
        }
      }
    ]
  }
}