Function remove_unnecessary_projections 

pub fn remove_unnecessary_projections(
    plan: Arc<dyn ExecutionPlan>,
) -> Result<Transformed<Arc<dyn ExecutionPlan>>>

This function checks if plan is a ProjectionExec, and inspects its input(s) to test whether it can push plan under its input(s). This function will operate on the entire tree and may ultimately remove plan entirely by leveraging source providers with built-in projection capabilities.