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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 (
    21085, 21086, 21087, 21088, 21089, 21090, 
    21091, 21092, 21093, 21094, 21095, 
    21096, 21097, 21098, 21099, 21100
  ) 
  AND cscart_product_review_prepared_data.storefront_id = 0

Query time 0.00016

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 (21085,21086,21087,21088,21089,21090,21091,21092,21093,21094,21095,21096,21097,21098,21099,21100)"
    }
  }
}