jnlsn

add_filter('posts_clauses', 'posts_clauses_with_tax', 10, 2);
function posts_clauses_with_tax( $clauses, $wp_query ) {
	global $wpdb;
	//array of sortable taxonomies
	$taxonomies = array('example-taxonomy', 'other-taxonomy');
	if (isset($wp_query->query['orderby']) && in_array($wp_query->query['orderby'], $taxonomies)) {
		$clauses['join'] .= "
			LEFT OUTER JOIN {$wpdb->term_relationships} AS rel2 ON {$wpdb->posts}.ID = rel2.object_id
			LEFT OUTER JOIN {$wpdb->term_taxonomy} AS tax2 ON rel2.term_taxonomy_id = tax2.term_taxonomy_id
			LEFT OUTER JOIN {$wpdb->terms} USING (term_id)