Bring Older Results with Long Polling

Question

I am creating a system and I want it to bring all the results already recorded in the database, but it only returns the recorded data from the moment I refresh the page, for example if I insert 10 records in the bank, the 10 will appear at the time I am on the page, but when I update, everyone disappears!

In the bank I have a table called longpolling and the columns id, notificacao (varchar(200)), and timestamp (timestamp).

Long Polling PHP script:

<?php
$timestart = time();
$pdo = new PDO('mysql:host=localhost;dbname=longpolling', 'root', '');

if(isset($_POST['timestamp'])){
    $timestamp = $_POST['timestamp'];
}else{
    $pega_time = $pdo->prepare("SELECT NOW() as now");
    $pega_time->execute();
    $row = $pega_time->fetchObject();

    $timestamp = $row->now;
}

$sql = $pdo->prepare("SELECT * FROM notificacoes WHERE timestamp > '$timestamp'");


$newData = false;
$notificacoes = array();

while(!$newData && (time()-$timestart)<20){
    $sql->execute();

    while($row = $sql->fetchAll(PDO::FETCH_ASSOC)){
        $notificacoes = $row;
        $newData = true;
    }

    usleep(500000);
}

$pega_time = $pdo->prepare("SELECT NOW() as now");
$pega_time->execute();
$row = $pega_time->fetchObject();

$timestamp = $row->now;
$data = array('notificacoes' => $notificacoes, 'timestamp' => $timestamp);
echo json_encode($data);
exit;

Script Jquery:

<script type="text/javascript">
$(function(){
    pegaNotificacoes();
});

function pegaNotificacoes(timestamp){
    var data = {};
    if(typeof timestamp != 'undefined') {
        data.timestamp = timestamp;
    }

    $.post('longpolling.php', data, function(res){

        for(i in res.notificacoes){
            $('#resultados').append(res.notificacoes[i].notificacao+'<br>')
        }

        pegaNotificacoes(res.timestamp);
    }, 'json');
}