Re: anti-join chosen even when slower than old plan

Tom Lane wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> > On Fri, Nov 12, 2010 at 4:15 AM, Cdric Villemain
> > <cedric(dot)villemain(dot)debian(at)gmail(dot)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.

Is there a TODO here?

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +