A different question then, Is there any meaning of discussing composite key not in the context of a table "primary key" ? 
For making additional hierarchy inside a single row by using columns.

On Mon, Nov 5, 2012 at 12:14 PM, Sylvain Lebresne <sylvain@datastax.com> wrote:
On Mon, Nov 5, 2012 at 10:53 AM, Maxim Veksler <maxim@vekslers.org> wrote:
Are indexes on composite column supported ?

Index on any part of the PRIMARY KEY is not supported (whether the PRIMARY KEY is composite or not btw), not yet at least.
If not, a suggestion for a work around?

In your case, you're asking for an index on the first component of the PRIMARY KEY, which is non-sensical in the sense that this component is already always "indexed".

If you want all the rows whose campaignId = 3, just do SELECT FROM userinfo2 WHERE campaignId = 3.