Re: index scan of whole table, can't see why

On 20 Jan 2005 at 9:34, Ragnar Hafstað wrote:

> On Wed, 2005-01-19 at 20:37 -0500, Dan Langille wrote:
> > Hi folks,
> >
> > Running on 7.4.2, recently vacuum analysed the three tables in
> > question.
> >
> > The query plan in question changes dramatically when a WHERE clause
> > changes from ports.broken to ports.deprecated. I don't see why.
> > Well, I do see why: a sequential scan of a 130,000 rows. The query
> > goes from 13ms to 1100ms because the of this. The full plans are at
> > http://rafb.net/paste/results/v8ccvQ54.html
> >
> > I have tried some tuning by:
> >
> > set effective_cache_size to 4000, was 1000
> > set random_page_cost to 1, was 4
> >
> > The resulting plan changes, but no speed improvment, are at
> > http://rafb.net/paste/results/rV8khJ18.html
> >
>
> this just confirms that an indexscan is not always better than a
> tablescan. by setting random_page_cost to 1, you deceiving the
> planner into thinking that the indexscan is almost as effective
> as a tablescan.
>
> > Any suggestions please?
>
> did you try to increase sort_mem ?

I tried sort_mem = 4096 and then 16384. This did not make a
difference. See http://rafb.net/paste/results/AVDqEm55.html

Thank you.
--
Dan Langille : http://www.langille.org/
BSDCan - The Technical BSD Conference - http://www.bsdcan.org/