Re: Calculation of unused columns

From: Tim Landscheidt pgsql-performance(at)postgresql(dot)org Re: Calculation of unused columns 2009-10-18 17:35:29 m3my3oppum.fsf@passepartout.tim-landscheidt.de Raw Message | Whole Thread | Download mbox 2009-10-17 22:58:16 from Volker Grabsch  2009-10-18 00:07:21 from Tom Lane   2009-10-18 01:41:34 from Tom Lane    2009-10-18 01:53:11 from Robert Haas     2009-10-18 02:19:33 from Tom Lane      2009-10-18 14:38:18 from Daniel Migowski       2009-10-18 15:59:56 from Tom Lane       2009-10-18 17:35:29 from Tim Landscheidt        2009-10-18 17:59:04 from Jeff Janes         2009-10-18 20:00:25 from Robert Haas          2009-10-18 20:54:10 from Tom Lane           2009-10-18 22:26:58 from Robert Haas            2009-10-18 23:41:38 from Tom Lane 📎             2009-10-19 16:01:17 from Tom Lane           2009-10-19 17:54:45 from Gerhard Wiesinger            2009-10-19 18:05:27 from Tom Lane    2009-10-19 17:36:15 from Simon Riggs     2009-10-19 17:43:20 from Tom Lane      2009-10-19 17:48:58 from Simon Riggs       2009-10-19 17:58:24 from Tom Lane        2009-10-19 18:09:27 from Simon Riggs         2009-10-19 18:23:45 from Tom Lane     2009-10-19 17:48:42 from "Kevin Grittner" pgsql-performance

Daniel Migowski <dmigowski(at)ikoffice(dot)de> wrote:

> I have a very common example which would illustrate the
> above problem a bit more. Guess the following view on a
> company table, which references the country of that company
> in another table. The view itself just returns the
> company-id and the country-name,

> create view companys_and_countries as
> select company.id, country.name from company left join
> country on (company.country_id = country.id);

> Pleaso note we have a left join here, so the contents of
> country do by no means affect the contents of the "id" row
> in that view. Lets see what happens when we just query for
> the ids:

> explain select id from companys_and_countries;

> The join is done anyway, even if its removed (At least on
> Postgres 8.3). [...]

How could that be done otherwise? PostgreSQL *must* look at
country to determine how many rows the left join produces.

Tim

Browse pgsql-performance by date

From Date Subject
Next Message Jeff Janes 2009-10-18 17:59:04 Re: Calculation of unused columns
Previous Message Jesper Krogh 2009-10-18 16:49:45 Full text search - query plan? PG 8.4.1