From: Tom Lane <tgl@sss.pgh.pa.us>
To: Claudio Freire <klaussfreire@gmail.com>
cc: Robert DiFalco <robert.difalco@gmail.com>,
 "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Efficiently query for the most recent record for a given user
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Comments: In-reply-to Claudio Freire <klaussfreire@gmail.com>
 message dated "Wed, 07 Aug 2013 15:39:45 -0300"
Date: Wed, 07 Aug 2013 15:04:09 -0400
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Claudio Freire <klaussfreire@gmail.com> writes:
> On Wed, Aug 7, 2013 at 3:34 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Note that there's no particular need to specify "desc" in the index
>> definition.  This same index can support searches in either direction
>> on the "called" column.

> Yeah, but it's faster if it's in the same direction, because the
> kernel read-ahead code detects sequential reads, whereas it doesn't
> when it goes backwards. The difference can be up to a factor of 10 for
> long index scans.

Color me skeptical.  Index searches are seldom purely sequential block
accesses.  Maybe if you had a freshly built index that'd never yet
suffered any inserts/updates, but in practice any advantage would
disappear very quickly after a few index page splits.

> Though... true... for a limit 1... it wouldn't matter that much.

That's the other point.

			regards, tom lane


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