| From: | "Nathan Boley" <npboley(at)gmail(dot)com> |
|---|---|
| To: | "Vladimir Sitnikov" <sitnikov(dot)vladimir(at)gmail(dot)com> |
| Cc: | "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "Robert Haas" <robertmhaas(at)gmail(dot)com>, "Gregory Stark" <stark(at)enterprisedb(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 00:12:35 |
| Message-ID: | 6fa3b6e20812111612m74555e44sfd3ef0ce5431ea9d@mail.gmail.com |
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| Lists: | pgsql-hackers |
>> What is the specific difference between what you are talking about and
>> what scalarineqsel already implements?
>
> Hmm... Northing new. Feel sorry for bothering you. I did not realize
> histograms are implemented.
>
Well, ISTM there is a profound difference. For scalarineqsel we care
about the total number of values in a bucket. For eqsel we care about
the total number of *distinct* values in each bucket ( which we don't
track ).
IMHO, the whole idea of increasing mcv's seems a mistake. Why not use
the limited storage in pg_statistic to try and estimate the
selectivity for ranges of values rather than a single value? That
gives way better coverage of the distribution. If the number of values
is too high to fit in a single bucket we put it in an mcv slot
anyways. *That* should be the mechanism by which the number of mcv's
increases.
I guess this is a bit off topic for the middle of a commit fest though.
-Nathan
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