| 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 18:31:57 |
| Message-ID: | 1229106717.8673.88.camel@ebony.2ndQuadrant |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Thread: | |
| Lists: | pgsql-hackers |
On Fri, 2008-12-12 at 13:18 -0500, Tom Lane wrote:
> I seem to recall Greg suggesting that there were ways to estimate
> ndistinct without sorting, but short of a fundamental algorithm change
> there's not going to be a win here.
Hash table? Haas Stokes suggests a Bloom filter.
Why not keep the random algorithm we have now, but scan the block into a
separate hash table for ndistinct estimation. That way we keep the
correct random rows for other purposes.
> > Right now we may as well use a random number generator.
>
> Could we skip the hyperbole please?
Some of the ndistinct values are very badly off, and in the common cases
I cited previously, consistently so.
Once I'm certain the rescue helicopter has seen me, I'll stop waving my
arms. (But yes, OK).
--
Simon Riggs www.2ndQuadrant.com
PostgreSQL Training, Services and Support
| From | Date | Subject | |
|---|---|---|---|
| Next Message | Greg Stark | 2008-12-12 18:33:12 | Re: benchmarking the query planner |
| Previous Message | Jeff Davis | 2008-12-12 18:26:58 | Re: Sync Rep: First Thoughts on Code |