Row-Reading Costs

When the database server needs to examine a row that is not already in memory, it must read that row from disk. The database server does not read only one row; it reads the entire page that contains the row. If the row spans more than one page, it reads all of the pages.

The actual cost of reading a page is variable and hard to predict. It is a combination of the following factors.

Factor

Effect of Factor

Buffering

If the needed page is in a page buffer already, the cost to read is nearly zero.

Contention

If two or more applications require access to the disk hardware, I/O requests can be delayed.

Seek time

The slowest thing that a disk does is to seek; that is, to move the access arm to the track that holds the data. Seek time depends on the speed of the disk and the location of the disk arm when the operation starts. Seek time varies from zero to nearly a second.

Latency

The transfer cannot start until the beginning of the page rotates under the access arm. This latency, or rotational delay, depends on the speed of the disk and on the position of the disk when the operation starts. Latency can vary from zero to a few milliseconds.

The time cost of reading a page can vary from microseconds for a page that is already in a buffer, to a few milliseconds when contention is zero and the disk arm is already in position, to hundreds of milliseconds when the page is in contention and the disk arm is over a distant cylinder of the disk.