Hi, Am 25.10.2012 um 20:22 schrieb Kevin Grittner: > > The idea is to model actual costs on your system. You don't show > your configuration or describe your hardware, but you show an > estimate of retrieving over 4000 rows through an index and describe a > response time of 4 seconds, so you must have some significant part of > the data cached. Sure my effective_cache_size 10 GB But my right Table has the size of 1.2 TB (yeah Terra) at the moment (partitioned a 40GB slices) and has 3 * 10^9 records My left table has only the size of 227MB and 1million records. Peanuts. > I would see how the workload behaves with the following settings: > > effective_cache_size = <your shared_buffers setting plus what the OS > shows as cached pages> > seq_page_cost = 1 > random_page_cost = 2 > cpu_tuple_cost = 0.05 > > You can set these in a session and check the plan with EXPLAIN. Try > various other important important queries with these settings and > variations on them. Once you hit the right factors to model your > actual costs, the optimizaer will make better choices without needing > to tinker with it each time. i've played with that already …. NESTED LOOP -> GOOD SEQSCAN -> VERY BAD SET random_page_cost = 4; 2012-08-14' AND '2012-08-30' -> NESTED LOOP 2012-08-13' AND '2012-08-30' -> SEQSCAN SET random_page_cost = 2; 2012-08-14' AND '2012-08-30' -> NESTED LOOP 2012-08-07' AND '2012-08-30' -> NESTED LOOP 2012-08-06' AND '2012-08-30' -> SEQSCAN SET random_page_cost = 1; 2012-08-14' AND '2012-08-30' -> NESTED LOOP 2012-08-07' AND '2012-08-30' -> NESTED LOOP 2012-07-07' AND '2012-08-30' -> NESTED LOOP 2012-07-06' AND '2012-08-30' -> SEQSCAN The thing is .. - You can alter what you want. The planner will switch at a certain time range. - There is not one case, where the SEQSCAN-Method will be better .. It's not possible. So the only way to tell the planner that he's doomed is SET enable_seqscan=0 which is not very elegant. (Query Hints would be BTW jehovah!) You would be forced to write something like this: var lastValueEnable_seqscan = "SHOw enable_seqscan" SET enable_seqscan=0; SELECT ... SET enable_seqscan=lastValueEnable_seqscan; Kind regards Andy -- Andreas Böckler andy@xxxxxxxxxxxx -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
