The Adaptive Server optimizer computes the cost of a clustered index partition scan differently, depending on the total number of pages that need to be scanned:

If the total number of pages that need to be scanned is less than or equal to two times the average size of a partition, the optimizer costs the scan as the total number of pages to be scanned divided by 2.

If the total number of pages that need to be scanned is greater than two times the average size of a partition, the optimizer costs the scan as the average number of pages in a partition.

The actual cost of the scan may be higher if:

The total number of pages that need to be scanned is less than the size of a partition, and

The data to be scanned lies entirely within one partition

If both of these conditions are true, the actual cost of the scan is the same as if the scan were executed serially.