private void forceSinglePassGrouping(Query query) {
        for (GroupingRequest groupingRequest : query.getSelect().getGrouping())
            forceSinglePassGrouping(groupingRequest.getRootOperation());
    }
When we only search a single node, doing all grouping in one pass is more efficient
container-search/src/main/java/com/yahoo/prelude/fastsearch/IndexedBackend.java