| From: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com> |
|---|---|
| To: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
| Cc: | pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: PATCH: adaptive ndistinct estimator v4 |
| Date: | 2015-04-15 06:45:55 |
| Message-ID: | CAMkU=1ySyCY1=8ZEeaEEPWD-9wn7ccXbQ6o=UJHU=3ZqA3-kxw@mail.gmail.com |
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| Lists: | pgsql-hackers pgsql-performance |
On Tue, Mar 31, 2015 at 12:02 PM, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com
> wrote:
> Hi all,
>
> attached is v4 of the patch implementing adaptive ndistinct estimator.
>
Hi Tomas,
I have a case here where the adaptive algorithm underestimates ndistinct by
a factor of 7 while the default estimator is pretty close.
5MB file:
https://drive.google.com/file/d/0Bzqrh1SO9FcETU1VYnQxU2RZSWM/view?usp=sharing
# create table foo2 (x text);
# \copy foo2 from program 'bzcat ~/temp/foo1.txt.bz2'
# analyze verbose foo2;
INFO: analyzing "public.foo2"
INFO: "foo2": scanned 6021 of 6021 pages, containing 1113772 live rows and
0 dead rows; 30000 rows in sample, 1113772 estimated total rows
WARNING: ndistinct estimate current=998951.78 adaptive=135819.00
Cheers,
Jeff
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