private static double rangedSelectivity(KllFloatsSketch kll, float val1, float val2) {
    float[] splitPoints = new float[] { val1, val2 };
    double[] boundaries = kll.getCDF(splitPoints, QuantileSearchCriteria.EXCLUSIVE);
    return boundaries[1] - boundaries[0];
  }
Given a RexCall & TableScan find max no of nulls. Currently it picks the
col with max no of nulls.

TODO: improve this

@param call
@param t
@return
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/stats/FilterSelectivityEstimator.java