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Re: Pet Peeves?

From: Octavio Alvarez <alvarezp(at)alvarezp(dot)ods(dot)org>
To: Gregory Stark <stark(at)enterprisedb(dot)com>
Cc: pgsql-general(at)postgresql(dot)org
Subject: Re: Pet Peeves?
Date: 2009-01-31 23:54:10
Message-ID: 1233446050.5647.73.camel@localhost.localdomain (view raw, whole thread or download thread mbox)
Lists: pgsql-general
On Sat, 2009-01-31 at 23:36 +0000, Gregory Stark wrote:
> Octavio Alvarez <alvarezp(at)alvarezp(dot)ods(dot)org> writes:
> > A crosstab is not but a presentational transform of the data set. Any
> > information you would eventually need can be taken from the original
> > data source, one way or another. That's why dynamic-column crosstab are
> > not a problem, and the DBMS should not worry about providing the
> > information about the columns, maybe by simply not allowing the
> > dynamic-column ones in subqueries.
> What about a WHERE clause like
> WHERE P1 > P2

You could either:

(1) do "FROM grades AS g1 INNER JOIN grades AS g2 ON g1.P1 > g2.P2",
generating the record set before applying the crosstab transformation.

(2) Since you are implying the existence of P1 and P2, use the
fixed-number-of-columns crosstab case instead, for which PG would
actually be able to give the column definitions without running the
query, and after that, joining the results with some other query.

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