Robert Haas <robertmhaas@gmail.com> writes:
> On Fri, Nov 12, 2010 at 4:15 AM, C�dric Villemain
> <cedric.villemain.debian@gmail.com> wrote:
>>> I wondering if we could do something with a formula like 3 *
>>> amount_of_data_to_read / (3 * amount_of_data_to_read +
>>> effective_cache_size) = percentage NOT cached. �That is, if we're
>>> reading an amount of data equal to effective_cache_size, we assume 25%
>>> caching, and plot a smooth curve through that point. �In the examples
>>> above, we would assume that a 150MB read is 87% cached, a 1GB read is
>>> 50% cached, and a 3GB read is 25% cached.
>> But isn't it already the behavior of effective_cache_size usage ?
> No.
I think his point is that we already have a proven formula
(Mackert-Lohmann) and shouldn't be inventing a new one out of thin air.
The problem is to figure out what numbers to apply the M-L formula to.
I've been thinking that we ought to try to use it in the context of the
query as a whole rather than for individual table scans; the current
usage already has some of that flavor but we haven't taken it to the
logical conclusion.
> The ideal of trying to know what is actually in cache strikes me as an
> almost certain non-starter.
Agreed on that point. Plan stability would go out the window.
regards, tom lane