SELECT 
  product_id 
FROM 
  cscart_mb_ali_products 
WHERE 
  product_id IN (
    21471, 21472, 21473, 21474, 21475, 21476, 
    21477, 21478, 21479, 21480, 21481, 
    21482, 21483, 21484, 21485, 21486
  )

Query time 0.00017

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 (21471,21472,21473,21474,21475,21476,21477,21478,21479,21480,21481,21482,21483,21484,21485,21486)",
      "using_index": true
    }
  }
}