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 -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
