private long getMaxNulls(RexCall call, HiveTableScan t) {
    long tmpNoNulls = 0;
    long maxNoNulls = 0;

    Set<Integer> iRefSet = HiveCalciteUtil.getInputRefs(call);
    List<ColStatistics> colStats = t.getColStat(new ArrayList<Integer>(iRefSet));

    for (ColStatistics cs : colStats) {
      tmpNoNulls = cs.getNumNulls();
      if (tmpNoNulls > maxNoNulls) {
        maxNoNulls = tmpNoNulls;
      }
    }

    return maxNoNulls;
  }
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