SELECT 
  product_id, 
  amount 
FROM 
  cscart_warehouses_sum_products_amount 
WHERE 
  1 
  AND product_id IN (521, 522, 523, 520)

Query time 0.00012

JSON explain

{
  "query_block": {
    "select_id": 1,
    "message": "Impossible WHERE noticed after reading const tables"
  }
}