Works basically as a normal Join operator with one exception: elements on the left input will be transfered even if there is no join partner. In this case the schema will be filled with null values.
See Join operator.
bidWnd = ELEMENTWINDOW({SIZE = 1}, bid) auctionWnd = ELEMENTWINDOW({SIZE = 1}, auction) out = LEFTJOIN({predicate='id=auction'}, auctionWnd, bidWnd) |