| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
| 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: | 2008-12-11 23:52:02 |
| Message-ID: | 13911.1229039522@sss.pgh.pa.us |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Thread: | |
| Lists: | pgsql-hackers |
Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> On Thu, 2008-12-11 at 22:29 +0000, Gregory Stark wrote:
>>> And I would like it even more if the sample size increased according
>>> to table size, since that makes ndistinct values fairly random for
>>> large tables.
>>
>> Unfortunately _any_ ndistinct estimate based on a sample of the table
>> is going to be pretty random.
> We know that constructed data distributions can destroy the
> effectiveness of the ndistinct estimate and make sample size irrelevant.
> But typical real world data distributions do improve their estimations
> with increased sample size and so it is worthwhile.
This is handwaving unsupported by evidence. If you've got a specific
proposal what to change the sample size to and some numbers about what
it might gain us or cost us, I'm all ears.
regards, tom lane
| From | Date | Subject | |
|---|---|---|---|
| Next Message | KaiGai Kohei | 2008-12-11 23:52:08 | Re: Updates of SE-PostgreSQL 8.4devel patches (r1268) |
| Previous Message | Bruce Momjian | 2008-12-11 23:50:19 | Re: benchmarking the query planner |