You still add one row. The column name is the remaining part of the composite key (repeat for each column) plus each of the column which is not in the composite key. I found it is much clearer to look at the data through Cassandra -cli which shows you how data is stored.
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Subject: CQL3 Compound Primary Keys - Do I have the right idea?
From: Adam Venturella <email@example.com>
Trying to better grasp compound primary keys and what they are conceptually doing under the hood. When you create a table with a compound primary key in cql3 (http://www.datastax.com/dev/blog/schema-in-cassandra-1-1) the first part of the key is the partition key. I get that and the subsequent parts help with the row name as I understand it.
So when you add a new row to that columnfamily/table, you are still adding a row. In other words, the RandomPartitioner places it somewhere in the cluster as a row on it's own as opposed to just adding a new column to an existing row, which would live on the same node as the row
The effect of the compound key means that those rows are effectively treated as if they were part of the same column, making it a wide column.
Is that the right idea or do I have the row / rp thing wrong?