将impala与odbc和php pdo连接，字符串字段为空

当我只使用odbc时，所有操作都会成功运行

$dsn = "DSN=DingdongImpala;host=172.168.1.100;port=21050;database=mmdb;";
$user = '';
$password = '';
$conn = odbc_connect($dsn, $user, $password);
$result = odbc_exec($conn, "select succount,failedcount,appid from t_mm_acc_date limit 1");
while($row = odbc_fetch_array($result)) {
    print_r($row);
}

结果是：

Array
(
    [succount] => 0          //int
    [failedcount] => 1       //int
    [appid] => 202361        //string
)

stdClass Object
(
    [succount] => 100       //int
    [failedcount] => 0      //int
    [appid] =>              //string, empty
)

但是当我使用pdo访问odbc时，字符串类型字段都是空的

$dsn = "odbc:DSN=DingdongImpala;Host=172.168.1.100;Port=21050;database=mmdb;";
$user = '';
$password = '';
$cnx = new PDO($dsn, $user, $password);
$result = $cnx->query("select succount,failedcount,appid from t_mm_acc_date limit 1");
print_r($result->fetchObject());

结果是：

Array
(
    [succount] => 0          //int
    [failedcount] => 1       //int
    [appid] => 202361        //string
)

stdClass Object
(
    [succount] => 100       //int
    [failedcount] => 0      //int
    [appid] =>              //string, empty
)

我尝试过很多情况，只要从impala中选择String字段，结果将为空，但int字段是正常的。
我的系统环境：

centos 6
PHP 5.3.6
php-odbc-5.3.3
unixODBC-2.2.14
ClouderaImpalaODBC-2.5.29.1009-1.el6.x86_64.rpm

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我面临同样的问题，作为解决方法，我将字符串字段强制转换为varchar，如下所示：

select succount, failedcount, cast(appid as varchar(255)) from t_mm_acc_date limit 1

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有人能帮我吗？