From: Mladen Gogala <mladen.gogala@vmsinfo.com>
To: Joshua Tolley <eggyknap@gmail.com>
Cc: Craig Ringer <craig@postnewspapers.com.au>
Cc: Neil Whelchel <neil.whelchel@gmail.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Slow count(*) again...
Date: Sun, 10 Oct 2010 23:14:43 -0400
Message-ID: <4CB28123.5080300@vmsinfo.com> (raw)
In-Reply-To: <4cb25a0e.2a978e0a.7797.33d6@mx.google.com>
References: <201010091626.18636.neil.whelchel@gmail.com>
<4CB11CC7.8070300@vmsinfo.com>
<4CB167CA.9070109@postnewspapers.com.au>
<4CB1F46E.80205@vmsinfo.com>
<4CB2410C.6010603@postnewspapers.com.au>
<4cb25a0e.2a978e0a.7797.33d6@mx.google.com>
On 10/10/2010 8:27 PM, Joshua Tolley wrote:
> It was asserted that reading bigger chunks would help performance; a response
> suggested that, at least in Linux, setting readahead on a device would
> essentially do the same thing. Or that's what I got from the thread, anyway.
> I'm interested to know how similar performance might be between the large
> block size case and the large readahead case. Comments, anyone?
>
Craig maybe right, the fact that Oracle is doing direct I/O probably
does account for the difference. The fact is, however, that the question
about slow sequential scan appears with some regularity on PostgreSQL
forums. My guess that a larger chunk would be helpful may not be
correct, but I do believe that there is a problem with a too slow
sequential scan. Bigger chunks are a very traditional solution which
may not work but the problem is still there.
--
Mladen Gogala
Sr. Oracle DBA
1500 Broadway
New York, NY 10036
(212) 329-5251
www.vmsinfo.com
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Subject: Re: Slow count(*) again...
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