Netflow-indexer is a program that uses xapian to index the flat file databases used by nfdump or flow-tools.

It lets you search a month of netflow data in seconds.  It doesn't index the
actual data, just what IP addresses are seen in which data file.  It is used in
a similar fashion as a bloom filter.

See the full documentation here: http://justinazoff.github.com/netflow-indexer/

netflow-indexer works fine and has no real issues that I am aware of, however 
Development effort has since switched over to https://github.com/justinazoff/flow-indexer which
uses go+leveldb instead of python+xapian and is much faster.