Robert Haas <robertmhaas@xxxxxxxxx> writes: > On Fri, Nov 12, 2010 at 4:15 AM, Cédric Villemain > <cedric.villemain.debian@xxxxxxxxx> 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 -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
