On Wed, Mar 17, 2010 at 9:01 PM, Eger, Patrick <peger@automotive.com> wrote:
> I'm running 8.4.2 and have noticed a similar heavy preference for
> sequential scans and hash joins over index scans and nested loops. Our
> database is can basically fit in cache 100% so this may not be
> applicable to your situation, but the following params seemed to help
> us:
>
> seq_page_cost = 1.0
> random_page_cost = 1.01
> cpu_tuple_cost = 0.0001
> cpu_index_tuple_cost = 0.00005
> cpu_operator_cost = 0.000025
> effective_cache_size = 1000MB
> shared_buffers = 1000MB
>
>
> Might I suggest the Postgres developers reconsider these defaults for
> 9.0 release, or perhaps provide a few sets of tuning params for
> different workloads in the default install/docs? The cpu_*_cost in
> particular seem to be way off afaict. I may be dead wrong though, fwiw
> =)
The default assume that the database is not cached in RAM. If it is,
you want to lower seq_page_cost and random_page_cost to something much
smaller, and typically make them equal. I often recommend 0.005, but
I know others have had success with higher values.
Ultimately it would be nice to have a better model of how data gets
cached in shared_buffers and the OS buffer cache, but that is not so
easy.
...Robert
