Re: Expected accuracy of planner statistics

["Jim C. Nasby" <jim(at)nasby(dot)net>]

On Thu, Sep 28, 2006 at 03:19:46PM -0700, Casey Duncan wrote:
> I have some databases that have grown significantly over time (as  
> databases do). As the databases have grown, I have noticed that the  
> statistics have grown less and less accurate. In particular, the  
> n_distinct values have become many OOM too small for certain foreign  
> key columns. Predictably this leads to poor query plans.

Search the -hackers archives. The problem is that you can't actually get
a good n_distinct estimate if you're sampling less than a very large
chunk of the table. Since our sampling maxes out at something like 30k
pages, at some point the n_distinct estimates just degrade. :(

Patches/solutions welcome. :)
-- 
Jim Nasby                                            jim(at)nasby(dot)net
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)