Which of the following is true about a Non-Unique Secondary Index (NUSI)?

A Non-Unique Secondary Index (NUSI) is indeed characterized by its allowance for duplicate values. This means that multiple entries in the indexed column can point to different rows. When you define a NUSI on a column, it enables efficient retrieval of rows that contain the same value in that column, as it provides rapid access to data without enforcing uniqueness constraints.

This feature makes NUSIs particularly useful when indexing columns where duplicates are common, as they enhance query performance without the overhead of enforcing uniqueness like a Unique Secondary Index would. The possibility of duplicates allows for more flexibility in how data can be organized and accessed, especially in scenarios where data redundancy is an inherent characteristic.

In contrast, the other options do not accurately describe the nature of a Non-Unique Secondary Index, since it does not prevent duplicates or uniquely identify rows and does not serve the purpose of partitioning rows. Thus, identifying the true characteristic of NUSIs as allowing duplicates is essential for understanding their role in Teradata systems.