private void forceSinglePassGrouping(GroupingOperation operation) {
        operation.setForceSinglePass(true);
        for (GroupingOperation childOperation : operation.getChildren())
            forceSinglePassGrouping(childOperation);
    }
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