| 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: | 2008-12-12 18:10:00 |
| Message-ID: | 7928.1229105400@sss.pgh.pa.us |
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Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> 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".
That's hardly the only thing we use ndistinct for. Also, it's a bit too
simplistic to suppose that we only have to make the right binary choice
between hash and something else at a particular plan level. If we don't
have at least ballpark-correct figures for cost and number of output
rows, we'll start making mistakes at higher plan levels.
regards, tom lane
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