| From: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Gregory Stark <stark(at)enterprisedb(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, "jd(at)commandprompt(dot)com" <jd(at)commandprompt(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, Greg Smith <gsmith(at)gregsmith(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: benchmarking the query planner |
| Date: | 2008-12-12 16:58:14 |
| Message-ID: | 1229101094.8673.44.camel@ebony.2ndQuadrant |
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| Lists: | pgsql-hackers |
On Fri, 2008-12-12 at 11:16 -0500, Tom Lane wrote:
> Perhaps a better plan is to try to de-emphasize use of ndistinct,
> though I concede I have no idea how to do that.
We don't actually care about the accuracy of the ndistinct much, just
the accuracy of our answer to the question "given work_mem = X, is it
better to use a hash plan".
So we just need to scan the table until we can answer that question
accurately enough. i.e. a variable sized sample.
Perhaps we could store a probability distribution for various values of
work_mem, rather than a single ndistinct value.
Anyway, definitely handwaving now to stimulate ideas.
--
Simon Riggs www.2ndQuadrant.com
PostgreSQL Training, Services and Support
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