| From: | Florian Pflug <fgp(at)phlo(dot)org> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Greg Stark <stark(at)mit(dot)edu>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: ANALYZE sampling is too good |
| Date: | 2013-12-12 22:25:22 |
| Message-ID: | D1C450BF-DFBF-4330-8C7B-5F8A69BE49D1@phlo.org |
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| Lists: | pgsql-hackers |
On Dec12, 2013, at 19:29 , Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> However ... where this thread started was not about trying to reduce
> the remaining statistical imperfections in our existing sampling method.
> It was about whether we could reduce the number of pages read for an
> acceptable cost in increased statistical imperfection.
True, but Jeff's case shows that even the imperfections of the current
sampling method are larger than what the n_distinct estimator expects.
Making it even more biased will thus require us to rethink how we
obtain a n_distinct estimate or something equivalent. I don't mean that
as an argument against changing the sampling method, just as something
to watch out for.
best regards,
Florian Pflug
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