On Tue, Feb 01, 2005 at 12:06:27AM -0500, Tom Lane wrote: > "Jim C. Nasby" <[EMAIL PROTECTED]> writes: > > On Mon, Jan 31, 2005 at 03:26:12PM -0500, Tom Lane wrote: > >> Preferably a whole lot of queries. All the measurement techniques I can > >> think of are going to have a great deal of noise, so you shouldn't > >> twiddle these cost settings based on just a few examples. > > > Are there any examples of how you can take numbers from pg_stats_* or > > explain analize and turn them into configuration settings (such and > > random page cost)? > > Well, the basic idea is to adjust random_page_cost so that the ratio of > estimated cost to real elapsed time (as shown by EXPLAIN ANALYZE) is the > same for seqscans and indexscans. What you have to watch out for is > that the estimated cost model is oversimplified and doesn't take into > account a lot of real-world factors, such as the activity of other > concurrent processes. The reason for needing a whole lot of tests is > essentially to try to average out the effects of those unmodeled > factors, so that you have a number that makes sense within the planner's > limited view of reality.
Given that, I guess the next logical question is: what would it take to collect stats on queries so that such an estimate could be made? And would it be possible/make sense to gather stats useful for tuning the other parameters? -- Jim C. Nasby, Database Consultant [EMAIL PROTECTED] Give your computer some brain candy! www.distributed.net Team #1828 Windows: "Where do you want to go today?" Linux: "Where do you want to go tomorrow?" FreeBSD: "Are you guys coming, or what?" ---------------------------(end of broadcast)--------------------------- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq