从本教程可以看出，rows.Closed()必须rows从哪里调用stmt.Query()，stmt.Closed()也应该stmt从db.Prepare()哪里调用？

// inside a function
stmt, err := db.Prepare(cmd) // cmd is SQL string
Check(err)
// should we add: defer stmt.Close()
rows, err := stmt.Query(params) // params is map/interface{}
defer rows.Close()
Check(err)