@Override
    public Optional<ColStatistics> estimate(List<ColStatistics> argStats) {
      PessimisticStatCombiner combiner = new PessimisticStatCombiner();
      combiner.add(argStats.get(1));
      combiner.add(argStats.get(2));
      return combiner.getResult();
    }
IF(expr1,expr2,expr3) <br>
If expr1 is TRUE (expr1 &lt;&gt; 0 and expr1 &lt;&gt; NULL) then IF() returns expr2;
otherwise it returns expr3. IF() returns a numeric or string value, depending
on the context in which it is used.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFIf.java