>> If you model the costing to reflect the reality on your server, good >> plans will be chosen. > > Wouldn't it be "better" to derive those costs from actual performance > data measured at runtime? > > Say, pg could measure random/seq page cost, *per tablespace* even. > > Has that been tried?
FWIW, awhile ago I wrote a simple script to measure this and found that the *actual* random_page / seq_page cost ratio was much higher than 4/1. The problem is that caching effects have a large effect on the time it takes to access a random page, and caching effects are very workload dependent. So anything automated would probably need to optimize the parameter values over a set of 'typical' queries, which is exactly what a good DBA does when they set random_page_cost... Best, Nathan -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
