Sequential access costs

Disk costs are lowest when the database server reads the rows of a table in physical order.

When the first row on a page is requested, the disk page is read into a buffer page. After the page is read in, it does not need not to be read again; requests for subsequent rows on that page are filled from the buffer until all the rows on that page are processed. When one page is exhausted, the page for the next set of rows must be read in.

When you use unbuffered devices for dbspaces, and the table is organized properly, the disk pages of consecutive rows are placed in consecutive locations on the disk. This arrangement allows the access arm to move very little when it reads sequentially. In addition, latency costs are usually lower when pages are read sequentially.