SELECT 
  SUM(amount) 
FROM 
  ushop_order_details 
WHERE 
  product_id = 314

Query time 0.00215

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "39.76"
    },
    "table": {
      "table_name": "ushop_order_details",
      "access_type": "ALL",
      "rows_examined_per_scan": 297,
      "rows_produced_per_join": 29,
      "filtered": "10.00",
      "cost_info": {
        "read_cost": "36.79",
        "eval_cost": "2.97",
        "prefix_cost": "39.76",
        "data_read_per_join": "6K"
      },
      "used_columns": [
        "product_id",
        "amount"
      ],
      "attached_condition": "(`marketplace`.`ushop_order_details`.`product_id` = 314)"
    }
  }
}