Re: 7.2.1 optimises very badly against 7.2

On Mon, 15 Jul 2002, Sam Liddicott wrote:

> > From: Martijn van Oosterhout [mailto:kleptog@svana.org]
> >
> > But if the planner chooses the seq scan two large
> > tables in parallel, the actual disk transfers degenerate to random access.
> > But only if they are on the same disk.
> >
> > Should postgres be worrying about this?
>
> I think it should.  The same applies if two different queries are running
> together of the same disk; which is probably any DB with allow_connections>1

Well, should it then worry about read-ahead? On most OSes, it
doesn't actually degenerate to 1-block random reads; it degerates
to something along the lines of 8-block random reads.

Trying to optimized based on more than the very simplest and common ideas
about physical layout opens up a huge can of worms when you don't actually
have any real control over or knowledge of that layout.

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