What are the challenges associated with indexing in NoSQL databases, especially with schema flexibility?

Schema flexibility in NoSQL databases presents challenges for indexing. Since the schema is not predefined, it's difficult to proactively create indexes for all possible query patterns. Denormalization (duplicating data) is often employed to improve query performance, but this requires careful consideration to maintain data consistency. Dynamic indexing mechanisms and query optimizers are essential for effectively leveraging indexes in these environments.