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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 (
    21069, 21070, 21071, 21072, 21073, 21074, 
    21075, 21076, 21077, 21078, 21079, 
    21080, 21081, 21082, 21083, 21084
  ) 
  AND cscart_product_review_prepared_data.storefront_id = 0

Query time 0.00012

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 (21069,21070,21071,21072,21073,21074,21075,21076,21077,21078,21079,21080,21081,21082,21083,21084)"
    }
  }
}