That's full new post tracking, which is a major slowdown if you have a large post table.
On version 6.4 (maybe 6.3) and higher, there's a control panel to turn off the full new post tracking.
WHERE B_Posted > ''.
Should have a timestamp, unless there's none for anonymouse users. Not sure if that'a bug or not... you didn't saw what version. But I think some bugs about full post tracking being on for anonymous users have been posted at www.ubbcentral.com