SELECT 
  cscart_products.* 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (
    20644, 20586, 20645, 20647, 20646, 20635, 
    20648, 20636, 20649, 20637, 20650, 
    20638, 20639, 20640, 20641, 20643, 
    21515, 20188, 20331
  )

Query time 0.00113

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_products",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "rows": 19,
          "filtered": 100,
          "index_condition": "cscart_products.product_id in (20644,20586,20645,20647,20646,20635,20648,20636,20649,20637,20650,20638,20639,20640,20641,20643,21515,20188,20331)"
        }
      }
    ]
  }
}