[_ck_]

>> The same performance gain is achieved using the existing forum_time index.

Ah so in that case, no ALTER required and there is zero extra storage required and the plugin can be used like this:

<?php

/*

Plugin Name: Topic Time Index

*/

add_filter('get_latest_topics_join','topic_time_index',99999);

function topic_time_index($join) {return " $join USE INDEX(forum_time) ";}

?>