Codeigniter error to execute update command in oracle database. DB_driver.php, Line Number: 330

when I am going to update my table in oracle db from codeigniter, I am getting an error,

Error Number: UPDATE BOTAB SET STATUS='y',TERMINAL='fazlay-pc',SLNO='101',REGNO='36' WHERE BOAC='1204260035198795'; Filename: C:\xampp\htdocs\agm_ci\system\database\DB_driver.php Line Number: 330

Yet my project is smoothly performing SELECT command from this table.

here is my model function.....

public function updateStatus($data)
{
    $update_data['regno'] = $data['regno'];
    $update_data['attendence'] = $data['attendence'];
    $update_data['terminal'] = $data['terminal'];
    $update_data['slno'] = $data['slno'];
    $update_data['index'] = $data['index'];

    $query = $this -> db -> query("UPDATE BOTAB SET STATUS='".$update_data['attendence']."',TERMINAL='".$update_data['terminal']."',SLNO='".$update_data['slno']."',REGNO='".$update_data['regno']."'  
    WHERE BOAC='".$update_data['index']."'; ");
}
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my db configuration is as follows.....

$db['default']['hostname'] = $tnsname;
$db['default']['username'] = 'XXXXX';
$db['default']['password'] = 'XXXXX';
$db['default']['database'] = '';
$db['default']['dbdriver'] = 'oci8';
$db['default']['dbprefix'] = '';
$db['default']['pconnect'] = TRUE;
$db['default']['db_debug'] = TRUE;
$db['default']['cache_on'] = FALSE;
$db['default']['cachedir'] = '';
$db['default']['char_set'] = 'utf8';
$db['default']['dbcollat'] = 'utf8_general_ci';
$db['default']['swap_pre'] = '';
$db['default']['autoinit'] = TRUE;
$db['default']['stricton'] = FALSE;
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Thanks in advance.