We currently define statistics in UnaryNode:
This has a few issues:
1. This can aggressively underestimate the size for Project. We assume each array/map has 100 elements, which is an overestimate. If the user projects a single field out of a deeply nested field, this would lead to huge underestimation. A safer sane default is probably 1.
2. It is not a property of UnaryNode to propagate statistics this way. It should be a property of Project.