From: | Chris Kratz <chris(dot)kratz(at)vistashare(dot)com> |
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To: | pgsql-performance(at)postgresql(dot)org |
Subject: | Re: Large time difference between explain analyze and normal run |
Date: | 2005-02-10 19:05:46 |
Message-ID: | 200502101405.46581.chris.kratz@vistashare.com |
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Lists: | pgsql-performance |
On Thursday 10 February 2005 01:58 pm, Tom Lane wrote:
> Chris Kratz <chris(dot)kratz(at)vistashare(dot)com> writes:
> > Does anyone have any idea why there be over a 4s difference between
> > running the statement directly and using explain analyze?
> >
> > Aggregate (cost=9848.12..9848.12 rows=1 width=0) (actual
> > time=4841.231..4841.235 rows=1 loops=1)
> > -> Seq Scan on answer (cost=0.00..8561.29 rows=514729 width=0)
> > (actual time=0.011..2347.762 rows=530576 loops=1)
> > Total runtime: 4841.412 ms
>
> EXPLAIN ANALYZE's principal overhead is two gettimeofday() kernel calls
> per plan node execution, so 1061154 such calls here. I infer that
> gettimeofday takes about 4 microseconds on your hardware ... which seems
> a bit slow for modern machines. What sort of box is it?
>
> regards, tom lane
OK, that makes sense.
Athlon XP 3000+
1.5G Mem
Is there a way to test the gettimeofday() directly?
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