| From: | Josh Berkus <josh(at)agliodbs(dot)com> |
|---|---|
| To: | pgsql-perform <pgsql-performance(at)postgresql(dot)org> |
| Cc: | pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: [HACKERS] Bad n_distinct estimation; hacks suggested? |
| Date: | 2005-04-25 19:18:26 |
| Message-ID: | 200504251218.27072.josh@agliodbs.com |
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| Lists: | pgsql-hackers pgsql-performance |
Guys,
> While it's not possible to get accurate estimates from a fixed size sample,
> I think it would be possible from a small but scalable sample: say, 0.1% of
> all data pages on large tables, up to the limit of maintenance_work_mem.
BTW, when I say "accurate estimates" here, I'm talking about "accurate enough
for planner purposes" which in my experience is a range between 0.2x to 5x.
--
--Josh
Josh Berkus
Aglio Database Solutions
San Francisco
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