| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Chris Kratz <chris(dot)kratz(at)vistashare(dot)com> |
| Cc: | pgsql-performance(at)postgresql(dot)org |
| Subject: | Re: Large time difference between explain analyze and normal run |
| Date: | 2005-02-10 18:58:37 |
| Message-ID: | 28553.1108061917@sss.pgh.pa.us |
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| Lists: | pgsql-performance |
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
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