Re: seq scan woes

On 7 Jun 2004 at 16:38, Rod Taylor wrote:

> On Mon, 2004-06-07 at 16:12, Dan Langille wrote:
> > On 7 Jun 2004 at 16:00, Rod Taylor wrote:
> >
> > > On Mon, 2004-06-07 at 15:45, Dan Langille wrote:
> > > > A production system has had a query recently degrade in performance.
> > > > What once took < 1s now takes over 1s. I have tracked down the
> > > > problem to a working example.
> > >
> > > What changes have you made to postgresql.conf?
> >
> > Nothing recently (ie. past few months). Nothing at all really.
> > Perhaps I need to start tuning that.
> >
> > > Could you send explain analyse again with SEQ_SCAN enabled but with
> > > nested loops disabled?
> >
> > See http://rafb.net/paste/results/zpJEvb28.html
>
> This doesn't appear to be the same query as we were shown earlier.

My apologies. I should try to cook dinner and paste at the same time.
;)

http://rafb.net/paste/results/rVr3To35.html is the right query.

> > > Off the cuff? I might hazard a guess that effective_cache is too low or
> > > random_page_cost is a touch too high. Probably the former.
> >
> > I grep'd postgresql.conf:
> >
> > #effective_cache_size = 1000 # typically 8KB each
> > #random_page_cost = 4 # units are one sequential page fetch cost
>
> This would be the issue. You haven't told PostgreSQL anything about your
> hardware. The defaults are somewhat modest.
>
> http://www.postgresql.org/docs/7.4/static/runtime-config.html
>
> Skim through the run-time configuration parameters that can be set in
> postgresql.conf.
>
> Pay particular attention to:
> * shared_buffers (you may be best with 2000 or 4000)
> * effective_cache_size (set to 50% of ram size if dedicated db
> machine)
> * random_page_cost (good disks will bring this down to a 2 from a
> 4)

I'll have a play with that and report back.

Thanks.
--
Dan Langille : http://www.langille.org/
BSDCan - http://www.bsdcan.org/