Which of these operations would not improve the performance of a parallel database system?

Storing data across several sites would not improve the performance of a parallel database system. Parallel database systems use multiple processors to handle large amounts of data simultaneously, and typically have multiple nodes or sites where data can be stored. Building indexes, deleting unwanted data, and evaluating queries are all operations that can help improve the performance of a parallel database system. However, storing data across several sites could potentially introduce more communication overhead and slow down the system, unless proper distributed database techniques are used to manage the data effectively. Therefore, storing data across several sites would not necessarily improve the performance of a parallel database system.