Function snapshot_generation 

pub fn snapshot_generation(expr: &Arc<dyn PhysicalExpr>) -> u64

Check the generation of this PhysicalExpr. Dynamic PhysicalExprs may have a generation that is incremented every time the state of the PhysicalExpr changes. If the generation changes that means this PhysicalExpr or one of its children has changed since the last time it was evaluated.

This algorithm will not produce collisions as long as the structure of the PhysicalExpr does not change and no PhysicalExpr decrements its own generation.