Parallel scans and large I/Os

The demand on buffer pools can become higher with parallel queries. With serial queries operating on the same pools, it is safe to assume that queries are issued at slightly different times and that the queries are in different stages of execution: some are accessing pages are already in cache, and others are waiting on I/O.

Parallel execution places different demands on buffer pools, depending on the type of scan and the degree of parallelism. Some parallel queries are likely to issue a large number of prefetch requests simultaneously.