Re: Calculation of unused columns

From: Tim Landscheidt <tim(at)tim-landscheidt(dot)de>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Calculation of unused columns
Date: 2009-10-18 17:35:29
Message-ID: m3my3oppum.fsf@passepartout.tim-landscheidt.de
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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

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