@Override
      public synchronized void update(byte[] region, byte[] row, S result) {
        sumVal = ci.add(sumVal, result);
      }
It sums up the value returned from various regions. In case qualifier is null, summation of all
the column qualifiers in the given family is done.
@param table table to scan.
@param ci    the user's ColumnInterpreter implementation
@param scan  the HBase scan object to use to read data from HBase
@return sum &lt;S&gt;
@throws Throwable The caller is supposed to handle the exception as they are thrown &amp;
                  propagated to it.
hbase-endpoint/src/main/java/org/apache/hadoop/hbase/client/coprocessor/AggregationClient.java