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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 (
    21021, 21022, 21023, 21024, 21025, 21026, 
    21027, 21028, 21029, 21030, 21031, 
    21032, 21033, 21034, 21035, 21036
  ) 
  AND cscart_product_review_prepared_data.storefront_id = 0

Query time 0.00014

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 (21021,21022,21023,21024,21025,21026,21027,21028,21029,21030,21031,21032,21033,21034,21035,21036)"
    }
  }
}