mapper.var.forbid-nulls has the same behavior as
This mapper function returns the variance of all the values found in each sliding window. The location and elevation returned are those associated with the most recent value in the sliding window.
If the number of values in the sliding window is greater than one, Bessel’s correction is applied.
mapper.var.forbid-nulls function can only be applied to values of type LONG or DOUBLE, when applied to STRING or BOOLEAN it does not return any value.
If a value is NULL in the bucket, the result is NULL.