On Wed, Feb 22, 2012 at 2:23 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> The industry accepted description for non-sequential access is "random
> access" whether or not the function that describes the movement is
> entirely random. To argue otherwise is merely hairsplitting.
I don't think so. For example, a bitmap index scan contrives to speed
things up by arranging for the table I/O to happen in ascending block
number order, with skips, rather than in random order, as a plain
index scan would do, and that seems to be a pretty effective
technique. Except to the extent that it interferes with the kernel's
ability to do readahead, it really can't be to read blocks 1, 2, 3, 4,
and 5 than to read blocks 1, 2, 4, and 5. Not reading block 3 can't
require more effort than reading it.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company