From opt_compression.mx:
@f opt_compression
@- The compressed database
Column stores mostly exploit some form of compression to reduce the
footprint on disk and to improve disk/memory/cache bandwidth at 
the expense of cpu/memory cycles. 
A naive approach is to take a (preferrably lightweight) compression
algorithm and deploy it against the units of access, e.g. disk/buffers.
In MonetDB our granularity is a BAT, which therefore forms the basis
for (de)compressed access.

I don't know whether this is still an actively developed idea. If it is, perhaps this can help:

http://code.google.com/p/snappy/

Roberto