reducing memory usage further

[ekg]

In the dmultimap, we use a bitvector to delimit sorted records and support rank/select queries.

Could the memory requirements for this be reduced if it were also built from disk backed structures, such as using https://github.com/stxxl/stxxl?

Once compressed, it is likely to be relatively small.

Another option would be to use DYNAMIC to build it in compressed space. This might have lower performance, but would be equivalent in terms of saving memory.