Sequential scans

When the database server performs a sequential scan of data or index pages, most of the I/O wait time is caused by seeking the appropriate starting page. To dramatically improve performance for sequential scans, you can bring in a number of contiguous pages with each I/O operation.

The action of bringing additional pages along with the first page in a sequential scan is called read ahead.

The timing of I/O operations that are needed for a sequential scan is also important. If the scan thread must wait for the next set of pages to be brought in after working its way through each batch, a delay occurs. Timing second and subsequent read requests to bring in pages before they are needed provides the greatest efficiency for sequential scans. The number of pages to bring in and the frequency of read-ahead I/O requests depends on the availability of space in the memory buffers. Read-ahead operations can increase page cleaning to unacceptable levels if too many pages are brought in with each batch or if batches are brought in too often.


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