Re: Index Scans become Seq Scans after VACUUM ANALYSE

On Wed, 24 Apr 2002, mlw wrote:

> I am not arguing about whether or not they do it, I am saying it is
> not always possible. I/O requests do not remain in queue waiting for
> reordering indefinitely.

It doesn't matter. When they go out to the disk they go out in
order. On every Unix-based OS I know of, and Novell Netware, if
you submit a single read request for consecutive blocks, those
blocks *will* be read sequentially, no matter what the system load.

So to get back to the original arugment:

> > >The supposed advantage of a sequential read over an random read, in
> > >an active multitasking system, is a myth. If you are executing one
> > >query and the system is doing only that query, you may be right.

No, it's very real, because your sequential read will not be broken up.

If you think it will, let me know which operating systems this
happens on, and how exactly it happens.

cjs
--
Curt Sampson <cjs(at)cynic(dot)net> +81 90 7737 2974 http://www.netbsd.org
Don't you know, in this new Dark Age, we're all light. --XTC