Re: Will Postgres ever lock with read only queries?

Chris <dmagick@gmail.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