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Re: Will Postgres ever lock with read only queries?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Chris <dmagick(at)gmail(dot)com>
Cc: Robert James <srobertjames(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Will Postgres ever lock with read only queries?
Date: 2009-07-28 01:40:36
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
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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