SELECT 
  product_id, 
  amount 
FROM 
  cscart_warehouses_sum_products_amount 
WHERE 
  1 
  AND product_id IN (
    20868, 20869, 20870, 20871, 20861, 20880, 
    20882
  )

Query time 0.00035

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_warehouses_sum_products_amount",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "rows": 7,
      "filtered": 100,
      "index_condition": "cscart_warehouses_sum_products_amount.product_id in (20868,20869,20870,20871,20861,20880,20882)"
    }
  }
}