Final answer:
In the context of databases, denormalization is considered undesirable in relational data models but is a common practice in Cassandra to improve read performance. Cassandra does not use foreign keys and promotes denormalization for speed and scalability.
Step-by-step explanation:
The feature that is undesirable in a relational data model but not in Cassandra is denormalization. In traditional relational database management systems (RDBMS), the process of normalization is often employed to reduce redundancy and improve data integrity. However, normalization can lead to complex joins and slower query performance. Conversely, Cassandra, which is a NoSQL database designed to handle large amounts of data across many commodity servers, actually benefits from denormalization. Denormalization in Cassandra improves read performance since Cassandra is optimized for writes and allows for faster reads at the cost of additional storage space and possible data duplication.
Foreign keys are also not used in Cassandra as it does not enforce referential integrity like a relational database does. Primary keys, however, are crucial in both relational models and Cassandra for uniquely identifying records. As for normalization, it's not commonly practiced in Cassandra due to its distributed nature and the goal to optimize for performance over strict data normalization rules.