Extended subscription table indexes for subscriber and subscribed to include the created field, which is used to sort for display. This lets us skip a filesort and do the join much more efficiently.
Alter table from 08to09.sql needs to be run manually (though no ill effects if you forget other than not getting the perf improvement).
fave_user_id_idx index changed from (user_id) to (user_id,modified), so the timestamp ordering can be done straight from the index while we're looking up the user's notices.
Added to 08to09.sql and 08to09_pg.sql; may need to be run manually by folks doing development.
(No harm if you don't update it, the favorites tab/rss feed will just stay inefficent.)
