What are the potential drawbacks of storing each user's viewed posts in a database for a PHP forum?

Storing each user's viewed posts in a database for a PHP forum can lead to increased database size and potential performance issues, especially as the number of users and posts grows. To address this, consider implementing a caching mechanism to store the viewed posts temporarily in memory or using browser cookies to track user interactions without constantly querying the database.

// Example of using browser cookies to track viewed posts
if(isset($_COOKIE['viewed_posts'])) {
    $viewed_posts = json_decode($_COOKIE['viewed_posts'], true);
} else {
    $viewed_posts = array();
}

// Add viewed post to the array
$viewed_posts[] = $post_id;

// Store the updated array in a cookie
setcookie('viewed_posts', json_encode($viewed_posts), time() + 86400, '/');