LeftOuterJoin

Like Join, but a left event also appears, through a separate selector, for the times when nothing matches it.

The output covers each left event’s whole lifetime. Where a matching right event overlaps it, the inner selector makes the payload; for the times when no matching right event is alive, the outer selector does. A left event is therefore cut into time slices.

In the diagram, a:1 [1,5) becomes a:1+none [1,2), a:1+10 [2,4) and a:1+none [4,5); b:2 [3,9) becomes [3,4) unmatched, [4,7) matched and [7,9) unmatched; a:3 [6,8) is unmatched throughout.

The first unmatched slice of a:1 and of b:2 is emitted as a start edge and an end edge: when it starts, Trill cannot yet know when a match will arrive.

See also: Join, WhereNotExists