RE: question about index cost estimates

> -----Original Message-----
> From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
> 
> "Hiroshi Inoue" <Inoue@tpf.co.jp> writes:
> > pages_fetched seems to be able to be greater than
> > baserel->pages. But if there's sufficiently large buffer
> > space pages_fetched would be <= baserel->pages.
> > Are there any assupmtions about buffer space ?
> 
> Right now cost_index doesn't try to account for that, because
> it doesn't have any way of knowing the relevant buffer-space
> parameter.  (As I said to Jeff, we have to consider kernel
> buffer space not just the number of Postgres shared buffers.)
> 
> cost_nonsequential_access does have a dependence on (a totally
> bogus estimate of) effective cache size, but it's a considerably
> weaker dependence than you suggest above.

Thanks.  I just confirmed my question because I didn't understand
whether effecive cache size is irrelevant to the calculation or not. 

> If we had a reliable
> estimate of cache size I'd be inclined to restructure this code
> quite a bit...
>

Yes,I know that reliable estimate is very significant but I have
no idea unfortunately.

Regards.

Hiroshi Inoue
Inoue@tpf.co.jp