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Re: Will Postgres ever lock with read only queries?

From: Nikolas Everett <nik9000(at)gmail(dot)com>
To: Robert James <srobertjames(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, 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 14:25:45
Message-ID: d4e11e980907280725x2067a914oa73bf294b9e4178b@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
Can you run those two queries with psql?

I remember having some trouble running multiple queries in the same pgadmin
process.  Both would get stuck until both finished I think.  I went to
running a pgadmin process per query.

On Tue, Jul 28, 2009 at 9:17 AM, Robert James <srobertjames(at)gmail(dot)com>wrote:

> 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: really stupid question about checkpointing
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Subject: Re: Will Postgres ever lock with read only queries?

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