If you are working on a large data set and are okay with a pretty good approximation, I highly recommend using the command:

nodetool --host <hostname> cfstats

This will dump out a list for each column family looking like this:

Column Family: widgets
SSTable count: 11
Space used (live): 4295810363
Space used (total): 4295810363
Number of Keys (estimate): 9709824
Memtable Columns Count: 99008
Memtable Data Size: 150297312
Memtable Switch Count: 434
Read Count: 9716802
Read Latency: 0.036 ms.
Write Count: 9716806
Write Latency: 0.024 ms.
Pending Tasks: 0
Bloom Filter False Postives: 10428
Bloom Filter False Ratio: 1.00000
Bloom Filter Space Used: 18216448
Compacted row minimum size: 771
Compacted row maximum size: 263210
Compacted row mean size: 1634

The "Number of Keys (estimate)" row is a good guess across the cluster and the performance is a lot faster than explicit count approaches.

If you are using an order-preserving partitioner, you can do this with get_range_slice or get_key_range.

If you are not, you will need to store your user ids in a special row.

I found an excellent article on this here.. http://www.planetcassandra.org/blog/post/counting-keys-in-cassandra

select count(*) from cf limit 1000000

Above statement can be used if we have an approximate upper bound known before hand. I found this useful for my case.

[Edit: This answer is out of date as of Cassandra 0.8.1 -- please see the Counters entry in the Cassandra Wiki for the correct way to handle Counter Columns in Cassandra.]

I m new to Cassandra, but I have messed around a lot with Google s App Engine. If no other solution presents itself, you may consider keeping a separate counter in a platform that supports atomic increment operations like memcached. I know that Cassandra is working on atomic counter increment/decrement functionality, but it s not yet ready for prime time.

I can only post one hyperlink because I m new, so for progress on counter support see the link in my comment below.

Note that this thread suggests ZooKeeper, memcached, and redis as possible solutions. My personal preference would be memcached.

http://www.mail-archive.com/user@cassandra.apache.org/msg03965.html

There is always map/reduce but that probably goes without saying. If you have that with hive or pig, then you can do it for any table across the cluster though I am not sure tasktrackers know about cassandra locality and so it may have to stream the whole table across the network so you get task trackers on cassandra nodes but the data they receive may be from another cassandra node :(. I would love to hear if anyone knows for sure though.

NOTE: We are setting up map/reduce on cassandra mainly because if we want an index later, we can map/reduce one into cassandra.