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Re: reuse a subquery

From: Charles Holleran <scorpdaddy(at)hotmail(dot)com>
To: <pgsql-novice(at)postgresql(dot)org>
Subject: Re: reuse a subquery
Date: 2010-08-09 14:34:21
Message-ID: BAY126-W20509EC1AE0CA930A3FA14CE940@phx.gbl (view raw or flat)
Thread:
Lists: pgsql-novice

 

> From: thom(at)linux(dot)com
> Date: Mon, 9 Aug 2010 15:12:51 +0100
> Subject: Re: [NOVICE] reuse a subquery
> To: scorpdaddy(at)hotmail(dot)com
> CC: pgsql-novice(at)postgresql(dot)org
> 
> On 9 August 2010 15:04, Charles Holleran <scorpdaddy(at)hotmail(dot)com> wrote:
> > I have a query that uses the same subquery twice.  What is the correct
> > syntax to reuse the subquery instead of running it twice?  The query below
> > 'works' but reruns the identical subquery.  The point of the subquery is to
> > limit the join work to the subset of table_a where c = 3 instead of the
> > entire table_a with c ranging from 0 to 65535.  The planner helps expedite
> > the rerun query, but there must be a better syntax for subquery reuse.
> >
> > E.g.
> >
> > SELECT *
> >
> > FROM
> > (
> >   SELECT *
> >   FROM table_a
> >   WHERE c = 3
> >   ORDER BY d
> > ) AS T1
> >
> > LEFT JOIN
> >
> > (
> >   SELECT *
> >   FROM table_a
> >   WHERE c = 3
> >   ORDER BY d
> > ) AS T2
> >
> > ON T2.d = (T1.d + 5)
> > WHERE T2.d IS NULL
> > ORDER BY T1.d;
> >
> 
> Can't you just do:
> 
> SELECT *
> FROM table_a
> WHERE d+5 NOT IN (SELECT t1.d FROM table_a AS t1)
> ORDER BY d;
> 
> -- 
> Thom Brown
> Registered Linux user: #516935
> 


The intent was to not run 2 selects from table_a.  If the subquery is reused, then only the one subquery selects from table_a, limiting the work thereafter to the limited subset of table_a where c = 3.  The second syntax suggested 'works' but also runs the select from table_a twice.

 

 
 		 	   		  

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Subject: Re: reuse a subquery
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