From: | "Hiroshi Inoue" <Inoue(at)tpf(dot)co(dot)jp> |
---|---|
To: | "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | "Jeff Hoffmann" <jeff(at)propertykey(dot)com>, <pgsql-hackers(at)postgresql(dot)org> |
Subject: | RE: question about index cost estimates |
Date: | 2000-05-18 07:49:44 |
Message-ID: | 000001bfc09d$a1c7fd60$2801007e@tpf.co.jp |
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> -----Original Message-----
> From: Tom Lane [mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us]
>
> "Hiroshi Inoue" <Inoue(at)tpf(dot)co(dot)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(at)tpf(dot)co(dot)jp
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