- The index is now on (cl_to, cl_type, cl_sortkey, cl_from) instead of (cl_to, cl_sortkey, cl_from), where cl_type is enum('page', 'subcat', 'file'). WHERE cl_to='Foo' ORDER BY cl_sortkey is thus now a filesort.
- cl_sortkey is now not human-readable, in principle. Currently it sort of is, but it can have a null in it. In the future, it will be totally binary, probably.