SELECT 
  product_id 
FROM 
  cscart_mb_ali_products 
WHERE 
  product_id IN (
    21021, 21022, 21023, 21024, 21025, 21026, 
    21027, 21028, 21029, 21030, 21031, 
    21032, 21033, 21034, 21035, 21036
  )

Query time 0.00015

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_mb_ali_products",
      "access_type": "range",
      "possible_keys": ["product_id"],
      "key": "product_id",
      "key_length": "4",
      "used_key_parts": ["product_id"],
      "rows": 16,
      "filtered": 100,
      "attached_condition": "cscart_mb_ali_products.product_id in (21021,21022,21023,21024,21025,21026,21027,21028,21029,21030,21031,21032,21033,21034,21035,21036)",
      "using_index": true
    }
  }
}