I don't know why, but about a week ago I started seeing a lot of errors like this in system.log:
9/15/12 5:55:21.769 AM Alfred[263]: DB Query: select item, count(item) as used from knowledge where ts > ? and item in (?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?) group by item
9/15/12 5:55:21.769 AM Alfred[263]: DB Error: 11 "database disk image is malformed"
It would occasionally take about a second for files to show up in the result list.
Deleting the Spotlight index didn't help, but then I noticed that the usage tab didn't display any statistics either:

I think I solved it by just pressing the reset knowledge button. But I didn't find search results for this issue, so I'm answering this question myself.