I just found in qa-config.php the option :QA_ALLOW_UNINDEXED_QUERIES with the note:
"Set QA_ALLOW_UNINDEXED_QUERIES to true if you don't mind running some database queries which
are not indexed efficiently. For example, this will enable browsing unanswered questions per
category. If your database becomes large, these queries could become costly."
I enabled it but nothing in the interface changed. So question is, how do I call the open questions per category? Whcih URL?