SELECT 
  product_id 
FROM 
  cscart_mb_ali_products 
WHERE 
  product_id IN (
    21005, 21006, 21007, 21008, 21009, 21010, 
    21011, 21012, 21013, 21014, 21015, 
    21016, 21017, 21018, 21019, 21020
  )

Query time 0.00014

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 (21005,21006,21007,21008,21009,21010,21011,21012,21013,21014,21015,21016,21017,21018,21019,21020)",
      "using_index": true
    }
  }
}