I think a more productive area of exploration (to reduce RAM usage) would be to make a StringFieldComparator that doesn't need full access to all terms data, ie, operates per segment yet only does a "few" ord lookups when merging the results across segments. If "few" is small enough we can just use us the seek-by-ord from the terms dict to do them. This would be a huge RAM reduction because we could then sort by string fields (eg "title" field) without needing all term bytes randomly accessible.