Well, this is an example from a showthread_query plugin I have on my site. I don't add to the two variables I mentioned though.
PHP Code:
$hook_query_fields = ",users.downloaded as downloaded, users.uploaded as uploaded";
$hook_query_joins = "LEFT JOIN " . TABLE_PREFIX . "users as users ON(user.userid = users.id)";
In this plugin, I added two new fields to be grabbed in the select statement and then added a join to another table in my database where those fields are from.