> Even though we all agree default_statistics_target = 10 is too low, > proposing a 40X increase in the default value requires more evidence > than this. In particular, the prospect of a 1600-fold increase in > the typical cost of eqjoinsel() is a mite scary.
I just did some very quick testing of a couple values for default_statistics_target on a database for work whose contents I am not at liberty to share. Suffice it to say it is a database with small tables, but a lot of the queries involve many joins. ANALYZE with default_statistics_target set to 10 takes 13 s. With 100, 92 s. With 1000, 289 s. I tried several representative queries and there was no measurable change in run-time. I compared a couple of the plans and the plans didn't change either. So then I said - gee, how much can I reduce this? I looked at two of the queries in more detail. The lowest default_statistics_target that produced the same plan for both queries was 5. Reducing the default_statistics_target all the way down to 1 changed the plans, but the new plans were just as fast as the old plans. Given the amount of clamor for a higher value for default_statistics_target, I'm a little surprised by these results. It may be that the queries I'm running are not ones for which more statistics generate better plans, but I think they really are representative of what we run. Maybe someone could suggest some types of query that would be likely to helped by better statistics? A random thought: maybe the reason I'm not seeing any benefit is because my tables are just too small - most contain at most a few thousand rows, and some are much smaller. Maybe default_statistics_target should vary with the table size? Something like, 0.1% of the rows to a maximum of 100... and then 0.01% of the rows after that to some higher maximum.... but always a minimum of at least 10. I'm guessing that people with really big tables are less likely to mind longer planning times and more likely to benefit from finding better plans... ...Robert -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
