The querycache table stores up to 1000 items from the miser'd special pages (currently Ancientpages, Deadendpages, Lonelypages, Longpages, Popularpages, Shortpages, Wantedpages) so that display of the special pages can be done quickly with pre-cached data and nice, navigable display. (You can change the number of items shown and scroll through, and wantedpages that are created ought to show as such though i've not tested this.)

The good: quick response and reasonable display of pages like: http://de.wikipedia.org/wiki/Spezial:Wantedpages http://es.wikipedia.org/wiki/Especial:Lonelypages http://meta.wikipedia.org/wiki/Special:Ancientpages

The bad: it takes a long time to rebuild all the caches for all the wikis. I've been running it for an hour and it's only up to 'fa' (doing wikipedia and wiktionary interleaved). For en.wikipedia it's sufficiently slow that they back up and really slug things down; I ended up canceling most of the runs on enwiki to keep things running smoothly.

The script to run the rebuilding is recache-all-specials in /home/wikipedia/bin (it's just a big mass URL hitter.)

So, while reading from the querycache is *great*, we could still use some improvements on making the original queries more efficient. Wantedpages and Lonelypages are particularly nasty on superhuge wikis.

-- brion vibber (brion @ pobox.com)