To: Nathan Boley <npboley@gmail.com>
cc: Jeff Janes <jeff.janes@gmail.com>, Jesper Krogh <jesper@krogh.cc>,
	pgsql-performance@postgresql.org
Subject: Re: reducing random_page_cost from 4 to 2 to force index scan 
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Comments: In-reply-to Nathan Boley <npboley@gmail.com>
	message dated "Mon, 16 May 2011 12:10:37 -0700"
Date: Mon, 16 May 2011 15:41:00 -0400
Message-ID: <20553.1305574860@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Nathan Boley <npboley@gmail.com> writes:
>> The accesses to an index are far more likely to be clustered than the
>> accesses to the underlying table, because the index is organized in a
>> way that's application-meaningful and the table not so much.

> So, to clarify, are you saying that if query were actually requesting
> rows uniformly random, then there would be no reason to suspect that
> index accesses would have hotspots? It seems like the index structure
> ( ie, the top node in b-trees ) could also get in the way.

The upper nodes would tend to stay in cache, yes, but we already assume
that in the index access cost model, in a kind of indirect way: the
model only considers leaf-page accesses in the first place ;-)

			regards, tom lane