Create a custom query 'orderby' type that uses data from other tables in WordPress

wp_custom_orderby.php

<?php
// Fetch data required for custom orderby types and left join it into the query
add_filter('posts_join_paged', 'custom_orderby_joiner', 10, 2);
function custom_orderby_joiner($join_paged_statement, $wp_query) {
  $orderby = $wp_query->query_vars['orderby'];

  if ($orderby === 'popularity')  {
    $join_paged_statement .= " INNER JOIN (SELECT postid, pageviews FROM wp_popularpostsdata) popularpostsdata ON popularpostsdata.postid = wp_posts.ID";
  }

  if ($orderby === 'rating')  {
    $join_paged_statement .= " INNER JOIN (SELECT pack_id, AVG(rating_value) as avg_rating FROM wp_art_ed_pro_pack_ratings GROUP BY pack_id) ratings ON ratings.pack_id = wp_posts.ID";
  }

  return $join_paged_statement;
}

// Apply custom orderby types
add_filter('posts_orderby', 'custom_orderby', 10, 2);
function custom_orderby($orderby_statement, $wp_query) {
  $order = $wp_query->query_vars['order'];
  $orderby = $wp_query->query_vars['orderby'];

  if ($orderby === 'popularity')  {
    $orderby_statement = "pageviews $order";
  }

  if ($orderby === 'rating')  {
    $orderby_statement = "avg_rating $order";
  }

  return $orderby_statement;
}

// Example usage
$the_query = new WP_Query(array(
  'orderby' => 'popularity', // or 'rating'
  'order'   => 'DESC'
));
?>