public void postMerge() {
        if (postMergeCompleted) return;
        root.postMerge(groupingLevels, firstLevel, 0);
        postMergeCompleted = true;
    }
Invoked after merging is done. It is intended used for resolving any dependencies or derivates
that might have changes due to the merge.
searchlib/src/main/java/com/yahoo/searchlib/aggregation/Grouping.java