| 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 16:43:01 |
| Message-ID: | 1229100181.8673.32.camel@ebony.2ndQuadrant |
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| Lists: | pgsql-hackers |
On Fri, 2008-12-12 at 11:16 -0500, Tom Lane wrote:
> If you want ANALYZE to be cheap then you simply don't get to have
> a trustworthy value of ndistinct.
Understood, but a cheap ANALYZE isn't always a higher priority than all
other considerations.
Solutions can also include
* allowing user to note that we would actually like to scan the whole
table (stats_target = -2?)
* manual mechanism for setting ndistinct that doesn't keep getting
overwritten by subsequent ANALYZEs
* have the executor do non-transactional update of the value of
ndistinct if it ever builds a hash table that is larger than expected
(simple learning optimizer action)
--
Simon Riggs www.2ndQuadrant.com
PostgreSQL Training, Services and Support
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