Re: merge>hash>loop

On Tue, Apr 18, 2006 at 06:26:48PM -0400, Tom Lane wrote:
> Markus Schaber <schabi(at)logix-tt(dot)com> writes:
> > Hmm, how does effective_cach_size correspond with it? Shouldn't a high
> > effective_cache_size have a similar effect?
>
> It seems reasonable to suppose that effective_cache_size ought to be
> used as a number indicating how much "stuff" would hang around from
> query to query. Right now it's not used that way...

Maybe it would be a reasonable first pass to have estimators calculate
the cost if a node found everything it wanted in cache and then do a
linear interpolation between that and the costs we currently come up
with? Something like pg_class.relpages / sum(pg_class.relpages) would
give an idea of how much of a relation is likely to be cached, which
could be used for the linear interpolation.

Of course having *any* idea as to how much of a relation was actually in
shared_buffers (or better yet, the OS cache) would be a lot more
accurate, but this simple method might be a good enough first-pass.
--
Jim C. Nasby, Sr. Engineering Consultant jnasby(at)pervasive(dot)com
Pervasive Software http://pervasive.com work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461