What can be a disadvantage of a Non-Unique Primary Index (NUPI)?

A Non-Unique Primary Index (NUPI) can lead to skewed data because it allows multiple rows to share the same index value. When multiple rows have the same index value, they will be stored in the same partition or block. If a significant number of rows in a table are assigned the same NUPI value, it can lead to an imbalanced distribution of data across the nodes of the database. This situation can create performance issues, such as an uneven workload among nodes, which may result in slower query performance or resource contention, thereby impacting overall system efficiency.

In contrast, a Unique Primary Index ensures that each row in the table has a distinct identifier, promoting even data distribution. Other options, like guaranteeing row uniqueness or disallowing duplicate values, do not apply to NUPI, since its primary characteristic is to allow duplicates. Additionally, while a NUPI might improve data access speed in some scenarios due to how data is clustered, this does not outweigh the potential disadvantage of data skew that can occur.