2010/11/12 Tom Lane <t...@sss.pgh.pa.us>: > Robert Haas <robertmh...@gmail.com> writes: >> On Fri, Nov 12, 2010 at 4:15 AM, Cédric Villemain >> <cedric.villemain.deb...@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.
Point is not to now the current cache, but like for ANALYZE on a regular basis (probably something around number of page read/hit) run a cache_analyze which report stats like ANALYZE do, and may be adjusted per table like auto_analyze is. -- Cédric Villemain 2ndQuadrant http://2ndQuadrant.fr/ PostgreSQL : Expertise, Formation et Support -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
