Per IRC discussion; as written this may not work correctly for UnusedCategories, as there may not be a category table entry.

For MostLinkedCategories this should work perfectly, though!

Special:MostLinkedCategories patch updated and applied in r103759

Comment on attachment 4781
proposed patch

Marking obsolete as patch half applied, and other half is wrong

Not sure how much brions statement is still true...

On enwiki:

mysql> explain select cat_title from category where cat_pages = 0;
+----+-------------+----------+------+---------------+-----------+---------+-------+--------+-------+

+----+-------------+----------+------+---------------+-----------+---------+-------+--------+-------+

+----+-------------+----------+------+---------------+-----------+---------+-------+--------+-------+
1 row in set (0.00 sec)