Date: Tue, 10 Dec 2013 15:40:09 -0500
From: Bruce Momjian <bruce@momjian.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: salah jubeh <s_jubeh@yahoo.com>,
 "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Explain analyze time overhead
Message-ID: <20131210204009.GA14636@momjian.us>
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 <14363.1386253334@sss.pgh.pa.us>
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On Thu, Dec  5, 2013 at 09:22:14AM -0500, Tom Lane wrote:
> salah jubeh <s_jubeh@yahoo.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@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +


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