Re: Bitmap Heap scan 8.1/8.2

2007/10/23, Martin Marques <martin(at)bugs(dot)unl(dot)edu(dot)ar>:
> Martin Marques escribió:
> > Pavel Stehule wrote:
> >>
> >> try
> >>
> >> set work_mem to '8MB';
> >> and
> >> explain analyze select ..
> >
> > These things didn't help. What changed the plan completely was this:
> >
> > seq_page_cost = 5.0 # measured on an arbitrary scale
> > cpu_tuple_cost = 0.05 # same scale as above
>
> Can someone explain how this parameters are measured? What is 5.0 in
> this case for seq_page_cost?
>

http://www.postgresql.org/docs/8.2/interactive/runtime-config-query.html

5.0 means so seq scan will be expensive for optimaliser, and
optimaliser will prefer index scan.