@Override
    protected void onDeserialize(Deserializer buf) {
        id = buf.getInt(null);
        byte tmp = buf.getByte(null);
        valid = (tmp != 0);
        tmp = buf.getByte(null);
        all = (tmp != 0);
        topN = buf.getLong(null);
        firstLevel = buf.getInt(null);
        lastLevel = buf.getInt(null);
        int numLevels = buf.getInt(null);
        for (int i = 0; i < numLevels; i++) {
            GroupingLevel level = (GroupingLevel) create(buf);
            groupingLevels.add(level);
        }
        root.deserializeWithId(buf);
    }
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