Re: Changing the random_page_cost default (was:

Tom Lane wrote:
> "Greg Sabino Mullane" <greg(at)turnstep(dot)com> writes:
>
>>N.B. My own personal starting default is 2, but I thought 3 was a nice
>>middle ground more likely to reach consensus here. :)
>
>
> Your argument seems to be "this produces nice results for me", not
> "I have done experiments to measure the actual value of the parameter
> and it is X". I *have* done experiments of that sort, which is where
> the default of 4 came from. I remain of the opinion that reducing
> random_page_cost is a band-aid that compensates (but only partially)
> for problems elsewhere. We can see that it's not a real fix from
> the not-infrequent report that people have to reduce random_page_cost
> below 1.0 to get results anywhere near local reality. That doesn't say
> that the parameter value is wrong, it says that the model it's feeding
> into is wrong.
>

I would like to second that. A while back I performed a number of
experiments on differing hardware and came to the conclusion that *real*
random_page_cost was often higher than 4 (like 10-15 for multi-disk raid
systems).

However I have frequently adjusted Pg's random_page_cost to be less than
4 - if it helped queries perform better.

So yes, it looks like the model is the issue - not the value of the
parameter!

regards

Mark