@Override
  public void accumulate(Object value) {
    if (value != null && value != QueryService.UNDEFINED) {
      distinct.addAll((Set) value);
    }
  }
The input data is the Set containing distinct values from each of the bucket nodes.
geode-core/src/main/java/org/apache/geode/cache/query/internal/aggregate/CountDistinctPRQueryNode.java