未定义索引PDO

我正在使用此代码

<?php
session_start();
include 'database.php';
if($_SERVER['REQUEST_METHOD'] == "POST"){
$username=$_POST['username'];
$pass=$_POST['password'];   
if(isset($username, $pass)) {
$pdo = Database::connect();
$sql= "SELECT * FROM users WHERE login_id = '$username' AND password = '$pass' ";
$query_res = $pdo->query($sql);
$count= count($query_res->fetchAll(PDO::FETCH_ASSOC));
if($count > 0){
        $_SESSION["username"] = $_POST["username"]; 
        //first_name
        $full_name_query= "SELECT first_name FROM users WHERE login_id = '$username'";
        $full_name_exec = $pdo->query($full_name_query);
        $_SESSION['full_name'] = $full_name_exec;
        //admin or user
        $role_query= "SELECT admin FROM users WHERE login_id = '$username'";
        $role_exec = $pdo->query($role_query);
        $_SESSION['role'] = $role_exec;

        header('Location: test/dashboard.php');
            }
        else
        {  

        echo 'incorrect';
        }
                                }
                                        }
?>

I use $_SESSION['full_name'] and $_SESSION['role'] in the next page after login

全名有效，但角色无效，我明白了

未定义索引：角色

我认为问题出在执行角色的第二个查询，关于多个查询的某些问题，不确定

并且，这是从登录用户的数据库中获取信息的最佳/正确方法吗？

感谢任何帮助！