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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 (
    21871, 21872, 21873, 21874, 21875, 21876, 
    21877, 21878, 21879, 21880, 21881, 
    21882, 21883, 21884, 21885, 21886
  ) 
  AND cscart_product_review_prepared_data.storefront_id = 0

Query time 0.00018

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 (21871,21872,21873,21874,21875,21876,21877,21878,21879,21880,21881,21882,21883,21884,21885,21886)"
    }
  }
}