Say we have a view like the following on collection A:

[... {$addFields: {s: "A"}}, {$unionWith: {coll: "B", pipeline: [... {$addFields: {s: "B"}}]}}]

And a predicate {s: "A"}- then we should be able to completely discard the $unionWith. Similarly, if we see {s: "B"}, we should only execute the inner pipeline of the $unionWith on collection "B". If we see {s: "C"}, we should EOF.