I'll add to travisturner's response - Pilosa has a few bells and whistles beyond being a straight up bitmap index including: - associating each bit with a time

I'll add to travisturner's response - Pilosa has a few bells and whistles beyond being a straight up bitmap index including: - associating each bit with a timestamp (at various granularities) and queries over time ranges. - adding arbitrary key/value metadata to each row or column - automatic sorting/caching of bitmaps to support "TopN" queries