| 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 |
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| 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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| Next Message | Bryce Nesbitt | 2008-11-02 17:51:21 | Seq scan on join, not on subselect? analyze this |
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