It looks like it was a miswritten query by one of our new sysops on fr.wikipedia.org. He has learned respect for the database the hard way. :)
(Slow query log lists it as taking 31141 seconds, which is presumably where it got cut off by a kill/restart. Ouch!)
I'll have to see if there's a way to have an automatic timeout/kill query for a certain mysql user; that would reduce the impact of these occasional accidents. (Sysop sql queries are sandboxed in a 'wikisql' user account which has limited privledges. In a super-ideal world, they'd probably also run on their own replicated server where they will never interfere with the live system. But, that means more computers. :)
-- brion vibber (brion @ pobox.com)