Skip site navigation (1) Skip section navigation (2)

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: 1657.1248745236@sss.pgh.pa.us (view raw or flat)
Thread:
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

In response to

Responses

pgsql-performance by date

Next:From: Matthew WakelingDate: 2009-07-28 09:45:01
Subject: Re: hyperthreaded cpu still an issue in 8.4?
Previous:From: ChrisDate: 2009-07-28 01:21:01
Subject: Re: Will Postgres ever lock with read only queries?

Privacy Policy | About PostgreSQL
Copyright © 1996-2014 The PostgreSQL Global Development Group