Figure 24-5 shows how a query scans a table that has three partitions on three physical disks. With a single engine, this query can benefit from parallel processing because one worker process can execute while others sleep waiting for I/O or waiting for locks held by other processes to be released. If multiple engines are available, the worker processes can run simultaneously. This configuration can yield high parallel performance by providing I/O parallelism.
Figure 24-5: Multiple worker processes access multiple partitions