Cheap direct updates

If Adaptive Server cannot perform an update in place, it tries to perform a cheap direct update—changing the row and rewriting it at the same offset on the page. Subsequent rows on the page are moved up or down so that the data remains contiguous on the page, but the row IDs remain the same. The pointers in the row offset table change to reflect the new locations.

A cheap direct update,must meet these requirements:

Cheap direct updates are almost as fast as in-place updates. They require the same amount of I/O, but slightly more processing. Two changes are made to the data page (the row and the offset table). Any changed index keys are updated by deleting old values and inserting new values. Cheap direct updates affect only indexes whose keys are changed by the update, since the page and row ID are not changed.