Subject: Re: [PERFORM] A Better External Sort?
From: "Jeffrey W. Baker" <jwbaker@acm.org>
To: Ron Peacetree <rjpeace@earthlink.net>
Cc: Josh Berkus <josh@agliodbs.com>, pgsql-hackers@postgresql.org,
	pgsql-performance@postgresql.org
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Date: Tue, 27 Sep 2005 10:26:28 -0700
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On Tue, 2005-09-27 at 13:15 -0400, Ron Peacetree wrote:

> That Btree can be used to generate a physical reordering of the data
> in one pass, but that's the weakest use for it.  The more powerful
> uses involve allowing the Btree to persist and using it for more
> efficient re-searches or combining it with other such Btrees (either as
> a step in task distribution across multiple CPUs or as a more efficient
> way to do things like joins by manipulating these Btrees rather than
> the actual records.)

Maybe you could describe some concrete use cases.  I can see what you
are getting at, and I can imagine some advantageous uses, but I'd like
to know what you are thinking.

Specifically I'd like to see some cases where this would beat sequential
scan.  I'm thinking that in your example of a terabyte table with a
column having only two values, all the queries I can think of would be
better served with a sequential scan.

Perhaps I believe this because you can now buy as much sequential I/O as
you want.  Random I/O is the only real savings.

-jwb