Optimizing a while loop that contains PDO query's - php

Question

Information
Currently building an notification page which lists all of the logged in users notifications which contain information about the notification on each one.
For example, without information
You have an unread message
With information
<Sarah> Sent you an message
Problem
Because the notifications require data such as Username (for message notifications) or a article title (say your following an author and they release a new blog post, one notification would need to pull username form users table and then also the title of the blog from the blog table) this causes my page to lag even on localhost which I'm guessing would get significantly worse once uploaded and tested in the wild.
Current Code
function showNotifications($userid){
$STH = $this->database->prepare('SELECT * FROM notifications WHERE user_id = :userid ORDER BY timestamp DESC');
$STH->execute(array(':userid' => $userid));
while($row = $STH->fetch(PDO::FETCH_ASSOC)){
$this->sortNotif($row);
}
Quick explanation about the function below, because I have different types of notifications I created a bunch of ID's for specific types, for example type 1 = new message, type 2 = new blog post
function sortNotif($notif){
switch ($notif['type']) {
case "1":
$msg = $this->getMessageData($notif['feature_id']);
$user = $this->userData($msg['sender']);
echo '<li><i>'.timeAgo($notif['timestamp']).'</i>'.$user['first_name'].' sent you a message</li>';
break;
}
}
As you can see for just showing that a user has a new message it creates 2 query's and once looped through 40 or so notifications, over 100 or so users becomes a strain on the server.
Final Words
If anyone needs more information please ask and I'll be sure to update this question asap, thanks!
Edit
Below are table structures as requested in the below comments.
notifications
id | user_id | feature_id | type | timestamp | read
users
id | username | password | first_name | last_name | email | verify_hash | avatar | type
messages
id | receiver | sender | replying_to | deleted | body | timestamp | read

Answer 1

Change of plan, since I misunderstood the set up.
You will want to pull all of the data from each of the 'type' tables in one go, rather than on a per notification basis. This means that you will need to loop through your notifications twice, once to grab all the ids and appropriate types and then a second time to output the results.
function showNotifications($userid){
$STH = $this->database->prepare('SELECT * FROM notifications WHERE user_id = :userid ORDER BY timestamp DESC');
$STH->execute(array(':userid' => $userid));
// Centralized Book keeping for the types.
// When you add a new type to the entire system, add it here as well.
$types = array();
// Add the first notification type
$types["1"] = array();
// "query" is pulling all the data you need concerning a notification
$types["1"]["query"] = "SELECT m.id, u.username, u.firstname FROM messages m, users u WHERE m.sender = u.id AND m.id IN ";
// "ids" will hold the relevant ids that you need to look up.
$types["1"]["ids"] = array();
// A second type, just for show.
// $types["2"] = array();
// $types["2"]["query"] = "SELECT a.id, u.username, u.firstname FROM articles a, users u WHERE a.sender = u.id AND a.id IN ";
// $types["2"]["ids"] = array();
// Use fetchAll to gather all of the notifications into an array
$notifications = $STH->fetchAll();
// Walk through the notifications array, placing the notification id into the corret
// "ids" array in the $types array.
for($i=0; $i< count($notifications); $i++){
$types[$notifications[$i]['type']]["ids"][] = $notifications[$i]['feature_id'];
}
// Walk through the types array, hit the database once for each type of notification that has ids.
foreach($types as $type_id => $type){
if(count($type["ids"]) > 0){
$STH = $this->database->prepare($type["query"] . "( " . implode(",", $type["ids"]) . " )");
$STH->execute();
// Creates a hash table with the primary key as the array key
$types[$type_id]['details'] = $STH->fetchAll(PDO::FETCH_GROUP|PDO::FETCH_ASSOC);
$types[$type_id]['details'] = array_map('reset', $types[$type_id]['details']);
// run array_map to make it easier to work with, otherwise it looks like this:
// $results = array(
// 1234 => array(0 => array('username' => 'abc', 'firstname' => '[...]')),
// 1235 => array(0 => array('username' => 'def', 'firstname' => '[...]')),
// );
}
}
// Now walk through notifications again and write out based on notification type,
// referencing $types[<notification type>]["details"][<message id>] for the details
for($i=0; $i< count($notifications); $i++){
// check to see if details for the specific notification exist.
if(isset($types[$notifications[$i]['type']]["details"][$notifications[$i]['feature_id']])){
$notification_details = $types[$notifications[$i]['type']]["details"][$notifications[$i]['feature_id']];
switch ($notifications[$i]['type']) {
case "1":
echo '<li><i>'.timeAgo($notifications[$i]['timestamp']).'</i>' . $notification_details['first_name'].' sent you a message</li>';
break;
}
}
}
}
Update : Added logic to skip a notification if no details are pulled (ex. either the message or the user were deleted)
I think you want to run a single query that gathers all the info either via joins or a more complicated where statement.
Option 1: this might need to be tweaked so as to not the cartesian product of the tables
SELECT n.id, n.type, m.id, m.body, u.username, u.first_name
FROM notifications n, messages m, users u
WHERE n.user_id = :userid AND m.id = n.feature_id AND u.id = m.sender
Option 2: if the table aliases don't work, then you'll need to replace them with the full table name
SELECT SELECT n.id, n.type, m.id, m.body, u.username, u.first_name
FROM notifications n
JOIN messages m
ON n.feature_id = m.id
JOIN users u
ON m.sender = u.id
WHERE n.user_id = :userid