@Override
  public SqlAggFunction getRollup() {
    return this;
  }
<code>Sum</code> is an aggregator which returns the sum of the values which
go into it. It has precisely one argument of numeric type (<code>int</code>,
<code>long</code>, <code>float</code>, <code>double</code>), and the result
is the same type.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/functions/HiveSqlSumAggFunction.java