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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 (
    20989, 20990, 20991, 20992, 20993, 20994, 
    20995, 20996, 20997, 20998, 20999, 
    21000, 21001, 21002, 21003, 21004
  ) 
  AND cscart_product_review_prepared_data.storefront_id = 0

Query time 0.00015

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 (20989,20990,20991,20992,20993,20994,20995,20996,20997,20998,20999,21000,21001,21002,21003,21004)"
    }
  }
}