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Re: Will Postgres ever lock with read only queries?

From: Robert James <srobertjames(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Chris <dmagick(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Will Postgres ever lock with read only queries?
Date: 2009-07-28 13:17:56
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
Thanks for the replies.  I'm running Postgres 8.2 on Windows XP, Intel Core
Duo (though Postgres seems to use only one 1 core).
The queries are self joins on very large tables, with lots of nested loops.

On Mon, Jul 27, 2009 at 9:40 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Chris <dmagick(at)gmail(dot)com> writes:
> > Robert James wrote:
> >> Hi.  I'm seeing some weird behavior in Postgres.  I'm running read only
> >> queries (SELECT that is - no UPDATE or DELETE or INSERT is happening at
> >> all).  I can run one rather complicated query and the results come
> >> back... eventually.  Likewise with another.  But, when I run both
> >> queries at the same time, Postgres seems to ground to a halt.
> > They're probably not blocking each other but more likely you're
> > exhausting your servers resources. If they return "eventually"
> > individually, then running both at the same time will take at least
> > "eventually x2".
> It could be a lot more than x2.  If the two queries together eat enough
> RAM to drive the machine into swapping, where it didn't swap while
> doing one at a time, the slowdown could be orders of magnitude.
> Watching vmstat output might be informative --- it would at least give
> an idea if the bottleneck is CPU, I/O, or swap.
>                        regards, tom lane

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Subject: Re: Will Postgres ever lock with read only queries?
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