Re: Explain analyze time overhead

On Thu, Dec 5, 2013 at 09:22:14AM -0500, Tom Lane wrote:
> salah jubeh <s_jubeh(at)yahoo(dot)com> writes:
> > When I excute a query, the exection time is about 1 minute; however, when I execute the query with explain analyze the excution time jumps to 10 minutes.
>
> This isn't exactly unheard of, although it sounds like you have a
> particularly bad case. Cheap commodity PCs tend to have clock hardware
> that takes multiple microseconds to read ... which was fine thirty years
> ago when that hardware design was set, but with modern CPUs that's
> painfully slow.
>
> Short of getting a better machine, you might look into whether you can run
> a 64-bit instead of 32-bit operating system. In some cases that allows
> a clock reading to happen without a context switch to the kernel.
>
> > This is a little bit starnge for me; did any one experience somthing like this? Can I trust the generated plans?
>
> The numbers are fine as far as they go, but you should realize that the
> relative cost of the cheaper plan nodes is being overstated, since the
> added instrumentation cost is the same per node call regardless of how
> much work happens within the node.

The original poster might also want to run pg_test_timing to get
hardware timing overhead:

http://www.postgresql.org/docs/9.3/static/pgtesttiming.html

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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