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Re: Subqueries

From: Gregory Stark <stark(at)enterprisedb(dot)com>
To: <ptufenkji(at)usj(dot)edu(dot)lb>
Cc: "'Helio Campos Mello de Andrade'" <helio(dot)campos(at)gmail(dot)com>, <pgsql-sql(at)postgresql(dot)org>
Subject: Re: Subqueries
Date: 2008-10-31 10:58:40
Message-ID: 87hc6tghen.fsf@oxford.xeocode.com (view raw or flat)
Thread:
Lists: pgsql-sql
"Pascal Tufenkji" <ptufenkji(at)usj(dot)edu(dot)lb> writes:

> I understand the fact that "the inner query is executed before the outer
> query and the inner query doesn't even know about the outer query."
>
> But why the following query can be executed, although the inner query is
> using the outer query.
>
> Aren't we here using the same concept ?

It's not that inner queries can't refer to outer queries. When they do it's
called a "correlated subquery" and it has to be executed once for every row of
the outer query.

It's that queries on one side of a join can't refer to tables on the other
side of the join.

-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Ask me about EnterpriseDB's On-Demand Production Tuning

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Subject: Seq scan on join, not on subselect? analyze this
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Subject: Re: Subqueries

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