Re: random_page_costs - are defaults of 4.0 realistic for SCSI RAID 1

Scott Marlowe wrote:
> On 9/10/07, Gregory Stark <stark(at)enterprisedb(dot)com> wrote:
>
>> "Luke Lonergan" <llonergan(at)greenplum(dot)com> writes:
>>
>>> Should be a lot higher, something like 10-15 is approximating accurate.
>>>
>> Most people's experience is that due to Postgres underestimating the benefits
>> of caching lowering the random_page_cost is helpful.
>>
> Quite often the real problem is that they have effective_cache_size
> too small, and they use random_page_cost to get the planner to switch
> to index scans on small tables. With a large effective_cache_size and
> small to moderate table (i.e. it fits in memory pretty handily) the
> planner seems much better in the last few major releases about picking
> an index over a sequential scan
In my case, I set effective_cache_size to 25% of the RAM available to
the system (256 Mbytes), for a database that was about 100 Mbytes or
less. I found performance to increase when reducing random_page_cost
from 4.0 to 3.0.

For a database that truly fits entirely in memory, I assume
random_page_cost is closer to 1.0. The planner should know that there is
no significant seek cost for RAM.

I will try to compare results tonight using 8.2. The last time I checked
may have been 8.1. I am also curious to see what the current algorithm
is with regard to effective_cache_size.

Cheers,
mark

--
Mark Mielke <mark(at)mielke(dot)cc>