Performance of count(*)

Hi,

I just try to find out why a simple count(*) might last that long.
At first I tried explain, which rather quickly knows how many rows
to check, but the final count is two orders of magnitude slower.

My MS_SQL server using colleague can't believe that.

$ psql InfluenzaWeb -c 'explain SELECT count(*) from agiraw ;'
                               QUERY PLAN
-----------------------------------------------------------------------
  Aggregate  (cost=196969.77..196969.77 rows=1 width=0)
    ->  Seq Scan on agiraw  (cost=0.00..185197.41 rows=4708941 width=0)
(2 rows)

real    0m0.066s
user    0m0.024s
sys     0m0.008s

$ psql InfluenzaWeb -c 'SELECT count(*) from agiraw ;'
   count
---------
  4708941
(1 row)

real    0m4.474s
user    0m0.036s
sys     0m0.004s


Any explanation?

Kind regards

          Andreas.

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