function learn_with_naw_set_random_thumbnail_for_existing_posts()
{
  $args = [
    'post_type' => 'post',
    'posts_per_page' => -1,
    'post_status' => 'publish',
    'meta_query' => [
      [
        'key' => '_thumbnail_id',
        'compare' => 'NOT EXISTS',
      ],
    ],
  ];

  $query = new WP_Query($args);

  if ($query->have_posts()) {
    while ($query->have_posts()) {
      $query->the_post();
      $attachments = get_posts([
        'post_type' => 'attachment',
        'post_mime_type' => 'image',
        'posts_per_page' => -1,
        'post_status' => 'inherit',
        'orderby' => 'rand',
      ]);

      if ($attachments) {
        $attachment = array_shift($attachments);
        set_post_thumbnail(get_the_ID(), $attachment->ID);
      }
    }
  }

  wp_reset_postdata();
}

// Hook this function to run on Init
add_action('init', 'learn_with_naw_set_random_thumbnail_for_existing_posts');