private Double computeFunctionSelectivity(RexCall call) {
    Double tmpNDV = getMaxNDV(call);
    if (tmpNDV == null) {
      // Could not be computed
      return null;
    }
    return 1 / tmpNDV;
  }
Selectivity of f(X,y,z) -> 1/maxNDV(x,y,z).
<p>
Note that = is considered a generic function and uses this method to find its selectivity.
@param call
@return
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/stats/FilterSelectivityEstimator.java