On Fri, Jul 9, 2010 at 6:13 AM, damien hostin <damien.hostin@xxxxxxxxx> wrote: >> Have you tried running ANALYZE on the production server? >> >> You might also want to try ALTER TABLE ... SET STATISTICS to a large >> value on some of the join columns involved in the query. > > Hello, > > Before comparing the test case on the two machines, I run analyse on the > whole and look at pg_stats table to see if change occurs for the columns. > but on the production server the stats never became as good as on the > desktop computer. I set statistic at 10000 on column used by the join, run > analyse which take a 3000000 row sample then look at the stats. The stats > are not as good as on the desktop. Row number is nearly the same but only 1 > or 2 values are found. > > The data are not balanced the same way on the two computer : > - Desktop is 12000 rows with 6000 implicated in the query (50%), > - "Production" (actually a dev/test server) is 6 million rows with 6000 > implicated in the query (0,1%). > Columns used in the query are nullable, and in the 5994000 other rows that > are not implicated in the query these columns are null. > > I don't know if the statistic target is a % or a number of value to obtain, It's a number of values to obtain. > but event set at max (10000), it didn't managed to collect good stats (for > this particular query). I think there's a cutoff where it won't collect values unless they occur significantly more often than the average frequency. I wonder if that might be biting you here: without the actual values in the MCV table, the join selectivity estimates probably aren't too good. > As I don't know what more to do, my conclusion is that the data need to be > better balanced to allow the analyse gather better stats. But if there is a > way to improve the stats/query with this ugly balanced data, I'm open to it > ! > > I hope that in real production, data will never be loaded this way. If this > appened we will maybe set enable_nestloop to off, but I don't think it's a > good solution, other query have a chance to get slower. Yeah, that usually works out poorly. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise Postgres Company -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
