SELECT 
  cscart_products.* 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (
    939, 937, 938, 940, 936, 3733, 3736, 3735, 
    3734
  )

Query time 0.00026

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_products",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "rows": 9,
      "filtered": 100,
      "index_condition": "cscart_products.product_id in (939,937,938,940,936,3733,3736,3735,3734)"
    }
  }
}