On Apr 29, 2011, at 10:25 AM, James Mansion <ja...@mansionfamily.plus.com> wrote: > Robert Haas wrote: >> The server can and does measure hit rates for the PG buffer pool, but to my >> knowledge there is no clear-cut way for PG to know whether read() is >> satisfied from the OS cache or a drive cache or the platter. >> >> > Does the server know which IO it thinks is sequential, and which it thinks is > random?
No. It models this in the optimizer, but the executor has no clue. And sometimes we model I/O as partly random, partly sequential, as in the case of heap fetches on a clustered index. So the answer isn't even a Boolean. ...Robert -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
