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SELECT 
  cscart_product_review_prepared_data.product_id AS product_id, 
  cscart_product_review_prepared_data.average_rating AS average_rating, 
  cscart_product_review_prepared_data.reviews_count AS reviews_count 
FROM 
  cscart_product_review_prepared_data 
WHERE 
  cscart_product_review_prepared_data.product_id IN (
    21391, 21392, 21393, 21394, 21395, 21396, 
    21397, 21398, 21399, 21400, 21401, 
    21402, 21403, 21404, 21405, 21406
  ) 
  AND cscart_product_review_prepared_data.storefront_id = 0

Query time 0.00018

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_review_prepared_data",
      "access_type": "ALL",
      "possible_keys": ["PRIMARY"],
      "rows": 5,
      "filtered": 100,
      "attached_condition": "cscart_product_review_prepared_data.storefront_id = 0 and cscart_product_review_prepared_data.product_id in (21391,21392,21393,21394,21395,21396,21397,21398,21399,21400,21401,21402,21403,21404,21405,21406)"
    }
  }
}