| From: | Mladen Gogala <mladen(dot)gogala(at)vmsinfo(dot)com> |
|---|---|
| To: | Joshua Tolley <eggyknap(at)gmail(dot)com> |
| Cc: | Craig Ringer <craig(at)postnewspapers(dot)com(dot)au>, Neil Whelchel <neil(dot)whelchel(at)gmail(dot)com>, "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: Slow count(*) again... |
| Date: | 2010-10-11 03:14:43 |
| Message-ID: | 4CB28123.5080300@vmsinfo.com |
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| Lists: | pgsql-hackers pgsql-performance |
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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