What is the point of column families?
I've just uncovered some interesting information from the RocksDB FAQ. (RocksDB is a K-V store.)

Here are some relevant extracts.


  Q: What are column families used for?
  
  A: The most common reasons of using column families: (1) use different
  compaction setting, comparators, compression types, merge operators,
  or compaction filters in different parts of data; (2) drop a column
  family to delete its data; (3) one column family to store metadata and
  another one to store the data.
  
  Q: What's the difference between storing data in multiple column
  family and in multiple rocksdb database?
  
  A: The main differences will be backup, atomic writes and performance
  of writes. The advantage of using multiple databases: database is the
  unit of backup or checkpoint. It's easier to copy a database to
  another host than a column family. Advantages of using multiple column
  families: (1) write batches are atomic across multiple column families
  on one database. You can't achieve this using multiple RocksDB
  databases. (2) If you issue sync writes to WAL, too many databases may
  hurt the performance.
  
  Q: I have different key spaces. Should I separate them by prefixes, or
  use different column families?
  
  A: If each key space is reasonably large, it's a good idea to put them
  in different column families. If it can be small, then you should
  consider to pack multiple key spaces into one column family, to avoid
  the trouble of maintaining too many column families.