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SELECT 
  rating_value, 
  COUNT(1) count 
FROM 
  cscart_product_reviews 
WHERE 
  `product_id` = 276708 
  AND `status` = 'A' 
GROUP BY 
  rating_value

Query time 0.00035

JSON explain

{
  "query_block": {
    "select_id": 1,
    "message": "no matching row in const table"
  }
}