Often the DB will show unacceptable time to reply to the ORDER BY stuff but will fetch comments by single ID without problems. But when that second part is a problem as well it is a good idea to use Redis as a "vulgar" cache for items as well, so that the recent stuff are probably into a Redis hash and you can fetch everything with a single pipelined MULTI/EXEC call (and fetch all the items returned as nils from the DB).