Robert,
> All true. I suspect that in practice the different between random and
> sequential memory page costs is small enough to be ignorable, although
> of course I might be wrong.
This hasn't been my experience, although I have not carefully measured
it. In fact, there's good reason to suppose that, if you were selecting
50% of more of a table, sequential access would still be faster even for
an entirely in-memory table.
As a parallel to our development, Redis used to store all data as linked
lists, making every object lookup effectively a random lookup. They
found that even with a database which is pinned in memory, creating a
data page structure (they call it "ziplists") and supporting sequential
scans was up to 10X faster for large lists.
So I would assume that there is still a coefficient difference between
seeks and scans in memory until proven otherwise.
--
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com
