| From: | "Jim C(dot) Nasby" <jim(at)nasby(dot)net> |
|---|---|
| To: | Casey Duncan <casey(at)pandora(dot)com> |
| Cc: | pgsql-general(at)postgresql(dot)org |
| Subject: | Re: Expected accuracy of planner statistics |
| Date: | 2006-09-29 01:00:11 |
| Message-ID: | 20060929010011.GQ34238@nasby.net |
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| Thread: | |
| Lists: | pgsql-general |
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)
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