From: Greg Smith <gsmith@gregsmith.com>
To: Alvaro Herrera <alvherre@commandprompt.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: I/O on select count(*)
Date: Wed, 14 May 2008 22:21:45 -0400 (EDT)
Message-ID: <Pine.GSO.4.64.0805142211300.5617@westnet.com> (raw)
In-Reply-To: <20080515020553.GN9838@alvh.no-ip.org>
References: <920841.71690.qm@web44808.mail.sp1.yahoo.com>
<482B1D5F.EE98.0025.0@wicourts.gov>
<Pine.GSO.4.64.0805142133250.27290@westnet.com>
<20080515020553.GN9838@alvh.no-ip.org>
On Wed, 14 May 2008, Alvaro Herrera wrote:
> If neither of the bits is set, then the transaction is either in
> progress (which you can check by examining the list of running
> transactions in shared memory) or your process is the first one to check
> (in which case, you need to consult pg_clog to know the status, and you
> can update the hint bits if you find out a permanent state).
So is vacuum helpful here because it will force all that to happen in one
batch? To put that another way: if I've run a manual vacuum, is it true
that it will have updated all the hint bits to XMIN_COMMITTED for all the
tuples that were all done when the vacuum started?
> Regarding FAQs, I'm having trouble imagining putting this in the user
> FAQ; I think it belongs into the developer's FAQ. However, a
> benchmarker is not going to look there. Maybe we should start "a
> benchmarker's FAQ"?
On the wiki I've started adding a series of things that are
performance-related FAQs. There's three of them mixed in the bottom of
http://wiki.postgresql.org/wiki/Frequently_Asked_Questions right now,
about slow count(*) and dealing with slow queries.
Here the FAQ would be "Why am I seeing all these writes when I'm just
doing selects on my table?", and if it's mixed in with a lot of other
performance related notes people should be able to find it. The answer
and suggestions should be simple enough to be useful to a user who just
noticed this behavior, while perhaps going into developer land for those
who want to know more about the internals.
--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD
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To: pgsql-performance@postgresql.org
Cc: gsmith@gregsmith.com, alvherre@commandprompt.com
Subject: Re: I/O on select count(*)
In-Reply-To: <Pine.GSO.4.64.0805142211300.5617@westnet.com>
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