Filtering on primary key with nullable column in cassandra versions 1.2.5 and greater

Hope that helps The primary keys directly determine how and where cassandra stores the data contained in a table (column family). The primary key consists of partition key and clustering key (optional).

Hope this helps There are two answers to your question here. One specific to your example, and a more general answer (which is probably what you are really after).
Answer for your example

this will help No, an index is not automatically created on the clustering column of a composite key.
If you created a secondary index on the clustering column you would actually suffer a performance hit when querying by it. Secondary indexes in Cassandra exist for convenience, not for performance. The bottom line is that they don't scale well, and a single query usually ends up having to hit several nodes in your cluster.