SELECT 
  rating_value, 
  COUNT(1) count 
FROM 
  cscart_product_reviews 
WHERE 
  `product_id` IN (23166) 
  AND `status` = 'A' 
GROUP BY 
  rating_value

Query time 0.00083

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "cscart_product_reviews.rating_value",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "cscart_product_reviews",
              "access_type": "ref",
              "possible_keys": ["idx_product_id"],
              "key": "idx_product_id",
              "key_length": "3",
              "used_key_parts": ["product_id"],
              "ref": ["const"],
              "rows": 1,
              "filtered": 100,
              "attached_condition": "cscart_product_reviews.product_id <=> 23166 and cscart_product_reviews.`status` = 'A'"
            }
          }
        ]
      }
    }
  }
}