Function dataflow::render::join::delta_join::build_update_stream

fn build_update_stream<G, Tr>(
    trace: Arranged<G, Tr>, 
    as_of: Antichain<G::Timestamp>, 
    source_relation: usize, 
    initial_closure: JoinClosure
) -> (Collection<G, Row>, Collection<G, DataflowError>) where
    G: Scope<Timestamp = Timestamp>,
    Tr: TraceReader<Time = G::Timestamp, Key = Row, Val = Row, R = Diff> + Clone + 'static,
    Tr::Batch: BatchReader<Tr::Key, Tr::Val, Tr::Time, Tr::R>,
    Tr::Cursor: Cursor<Tr::Key, Tr::Val, Tr::Time, Tr::R>, 

Builds the beginning of the update stream of a delta path.

At start-up time only the delta path for the first relation sees updates, since any updates fed to the other delta paths would be discarded anyway due to the tie-breaking logic that avoids double-counting updates happening at the same time on different relations.