WordPress fix for slow non-subscriber user query.

fix.php

<?php
// Fix non-subscriber query for large data
function kw_adjust_for_non_subscribers($args) {
	global $wpdb;
	if ($args['join'] == ' INNER JOIN '.$wpdb->usermeta.' ON (katw_wp_users.ID = '.$wpdb->usermeta.'.user_id)' AND
		$args['where'] == ' AND ( ('.$wpdb->usermeta.'.meta_key = \'katw_wp_user_level\' AND CAST('.$wpdb->usermeta.'.meta_value AS CHAR) != \'0\') )') {
		// Switch to RIGHT JOIN
		$args['join'] = ' RIGHT JOIN '.$wpdb->usermeta.' ON (katw_wp_users.ID = '.$wpdb->usermeta.'.user_id)';
	}
	
	return($args);
}
add_filter( 'get_meta_sql', 'kw_adjust_for_non_subscribers');

MySQL doesn't like the INNER JOIN that WordPress builds while looking for that info. Switching it to a RIGHT JOIN gets the same results, and a much better execution plan. (for 2 million users, it was 4s instead of 91s). This is definitely a hacky fix.