Function is_volatile 

pub fn is_volatile(expr: &Arc<dyn PhysicalExpr>) -> bool

Returns true if the expression is volatile, i.e. whether it can return different results when evaluated multiple times with the same input.

For example the function call RANDOM() is volatile as each call will return a different value.

This method recursively checks if any sub-expression is volatile, for example 1 + RANDOM() will return true.