Re: Big problem with sql update operation

Michal Szymanski wrote:
> There is another strange thing. We have two versions of our test
> >>environment one with production DB copy and second genereated with
> >>minimal data set and it is odd that update presented above on copy of
> >>production is executing 170ms but on small DB it executing 6s !!!!
> >
> >How are you vacuuming the tables?
> >
> Using pgAdmin (DB is installed on my laptop) and I use this tool for
> vaccuminh, I do not think that vaccuming can help because I've tested on
> both database just after importing.

I think you are misunderstanding the importance of vacuuming the table.
Try this: on a different terminal from the one running the test, run a
VACUUM on the updated table with vacuum_cost_delay set to 20, on an
infinite loop.  Keep this running while you do your update test.  Vary
the vacuum_cost_delay and measure the average/min/max UPDATE times.
Also try putting a short sleep on the infinite VACUUM loop and see how
its length affects the UPDATE times.

One thing not clear to me is if your table is in a clean state.  Before
running this test, do a TRUNCATE and import the data again.  This will
get rid of any dead space that may be hurting your measurements.

--
Alvaro Herrera                        http://www.advogato.org/person/alvherre
"The Postgresql hackers have what I call a "NASA space shot" mentality.
 Quite refreshing in a world of "weekend drag racer" developers."
(Scott Marlowe)