Re: seq scan woes

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.

> > 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)

--
Rod Taylor <rbt [at] rbt [dot] ca>

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