| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Gregory Stark <stark(at)enterprisedb(dot)com> |
| Cc: | Simon Riggs <simon(at)2ndQuadrant(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 16:16:16 |
| Message-ID: | 6558.1229098576@sss.pgh.pa.us |
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Gregory Stark <stark(at)enterprisedb(dot)com> writes:
> For that matter, if we do consider sampling 5% of the table we may as well
> just go ahead and scan the whole table. It wouldn't take much longer and it
> would actually produce good estimates.
Yeah. Anything over a small fraction of a percent is going to imply
fetching every page anyway, for typical row widths. If you want ANALYZE
to be cheap then you simply don't get to have a trustworthy value of
ndistinct.
Perhaps a better plan is to try to de-emphasize use of ndistinct,
though I concede I have no idea how to do that.
regards, tom lane
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