| From: | Sergey Koposov <koposov(at)ast(dot)cam(dot)ac(dot)uk> |
|---|---|
| To: | Peter Geoghegan <peter(at)2ndquadrant(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: bitmap scan much slower than index scan, hash_search_with_hash_value |
| Date: | 2012-09-02 16:10:13 |
| Message-ID: | alpine.LRH.2.02.1209021700010.25232@calx046.ast.cam.ac.uk |
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| Lists: | pgsql-hackers |
On Sun, 2 Sep 2012, Peter Geoghegan wrote:
> On 2 September 2012 16:26, Sergey Koposov <koposov(at)ast(dot)cam(dot)ac(dot)uk> wrote:
>
> That's why we support altering that value with an ALTER TABLE...ALTER
> COLUMN DDL statement. You might at least consider increasing the
> statistics target for the column first though, to see if that will
> make the n_distinct value better accord with reality.
Thanks, I've forgot about that possibility.
After fixing the n_distinct to the correct value the index scan became the
preferred option. and the row number estimate for the index scan became
more realistic
Sorry for the noise.
Sergey
*****************************************************
Sergey E. Koposov, PhD, Research Associate
Institute of Astronomy, University of Cambridge
Madingley road, CB3 0HA, Cambridge, UK
Tel: +44-1223-337-551 Web: http://www.ast.cam.ac.uk/~koposov/
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