public State terminatePartial() {
      return state;
    }
Note that percentile can be null in a global aggregation with
 0 input rows:  "select percentile(col, 0.5) from t where false"
 In that case, iterate(null, null) will be called once.
ql/src/java/org/apache/hadoop/hive/ql/udf/UDAFPercentile.java