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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 (
    21519, 21520, 21521, 21522, 21523, 21524, 
    21525, 21526, 21527, 21528, 21529, 
    21530, 21531, 21532, 21533, 21534
  ) 
  AND cscart_product_review_prepared_data.storefront_id = 0

Query time 0.00029

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 (21519,21520,21521,21522,21523,21524,21525,21526,21527,21528,21529,21530,21531,21532,21533,21534)"
    }
  }
}