@Override
    public int hashCode() {
        return super.hashCode() + id + (valid ? 66 : 99) + (all ? 666 : 999) + (int)topN + groupingLevels.hashCode();
    }
Returns whether ordering will need results collected in children.
In that case we will probably just do a single pass.
searchlib/src/main/java/com/yahoo/searchlib/aggregation/Grouping.java