public long calculateFeatureCount() {
        return kIndexBuilder.values().stream()
                .map(index -> index.map.keySet())
                .reduce(
                        new HashSet<>(),
                        (acc, keySet) -> {
                            keySet.forEach(acc::add);
                            return acc;
                        }, (acc1, acc2) -> {
                            acc1.addAll(acc2);
                            return acc1;
                        })
                .size();
    }
@return The number of unique features in index.
predicate-search/src/main/java/com/yahoo/search/predicate/index/conjunction/ConjunctionIndexBuilder.java