Re: Large time difference between explain analyze and normal run - Mailing list pgsql-performance

Chris Kratz <chris.kratz@vistashare.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